Microsoft began the process of clamping down on high-volume mailboxes earlier this year. The process is still rolling out as Microsoft gradually imposes the limit (which was always documented but not enforced). Mailboxes which receive more than 3,600 messages in an hour are blocked from receiving further messages for an hour and senders receive a non-delivery notification with code 5.2.122. This limit applies to messages received from any source, including people in the same tenant, and is designed to stop Exchange Online mailboxes filling up because of problems with automated mailers, like those used by applications to notify people about the progress of jobs.
Clamping Down on Individual Senders
In message center notification MC272450 posted July 23, Microsoft says that they will introduce a further restriction in September 2021. Like the previous restriction, the reason specified is to block single-sender mail storms and deter DoS attacks. The big difference is that the focus moves from messages coming from all sources to messages coming from a specific external source. To do this, Exchange Online tracks sender-recipient pairs (SRPs). As messages arrive in a mailbox, Exchange Online notes the sender and builds a table of SRPs. If a single sender sends more than 33% of the overall threshold (1,200 of 3,600), Exchange Online stops accepting messages from that sender to the mailbox for an hour. The mailbox continues to receive messages from other senders.
The exception is that the limit does not apply to messages sent from an Exchange Online mailbox in the same tenant. The limit does apply to:
Messages from Exchange on-premises mailboxes in the same tenant.
Exchange Online messages from other Office 365 tenants.
Messages from any other email system.
Blocked senders will receive non-delivery notifications with a 5.2.121 code. The mailbox owner will get a message to tell them that their mailbox has stopped receiving messages from the sender for an hour (the countdown starts once Exchange Online detects the problem). and administrators will be able to see the affected mailboxes in the Mailbox exceeding receiving limits report (aka Hot Recipients) in the Reports (Mail Flow) section of the Exchange admin center (Figure 1).
Figure 1: If your Exchange Online tenant has any hot recipients, they’ll turn up in this report
Microsoft suggests that Exchange administrators ask the mailbox owners why they receive so much email from the blocked sender.
Time to Review Email Generation by Applications
Microsoft says that this change will stop a malicious user blocking email flow to a mailbox. In other words, an attacker can’t try to stop someone receiving legitimate email by generating a flood of email to their mailbox. They also note that only a small percentage of mailboxes hut the SRP limit currently. My feeling is that the origins of most email likely to be blocked by the SRP limit come from applications generating frequent notifications and other updates. The tightening of the overall limit and the introduction of the new SRP limit is a good wake-up call for organizations to review the rate of email generation by applications and remove messages which are not strictly necessary.
So much change, all the time. It’s a challenge to stay abreast of all the updates Microsoft makes across Office 365. Subscribe to the Office 365 for IT Pros eBook to receive monthly insights into what’s happening.
{"id":null,"mode":"button","open_style":"in_modal","currency_code":"EUR","currency_symbol":"\u20ac","currency_type":"decimal","blank_flag_url":"https:\/\/office365itpros.com\/wp-content\/plugins\/tip-jar-wp\/\/assets\/images\/flags\/blank.gif","flag_sprite_url":"https:\/\/office365itpros.com\/wp-content\/plugins\/tip-jar-wp\/\/assets\/images\/flags\/flags.png","default_amount":100,"top_media_type":"featured_image","featured_image_url":"https:\/\/office365itpros.com\/wp-content\/uploads\/2022\/11\/cover-141x200.jpg","featured_embed":"","header_media":null,"file_download_attachment_data":null,"recurring_options_enabled":true,"recurring_options":{"never":{"selected":true,"after_output":"One time only"},"weekly":{"selected":false,"after_output":"Every week"},"monthly":{"selected":false,"after_output":"Every month"},"yearly":{"selected":false,"after_output":"Every year"}},"strings":{"current_user_email":"","current_user_name":"","link_text":"Virtual Tip Jar","complete_payment_button_error_text":"Check info and try again","payment_verb":"Pay","payment_request_label":"Office 365 for IT Pros","form_has_an_error":"Please check and fix the errors above","general_server_error":"Something isn't working right at the moment. Please try again.","form_title":"Office 365 for IT Pros","form_subtitle":null,"currency_search_text":"Country or Currency here","other_payment_option":"Other payment option","manage_payments_button_text":"Manage your payments","thank_you_message":"Thank you for supporting the work of Office 365 for IT Pros!","payment_confirmation_title":"Office 365 for IT Pros","receipt_title":"Your Receipt","print_receipt":"Print Receipt","email_receipt":"Email Receipt","email_receipt_sending":"Sending receipt...","email_receipt_success":"Email receipt successfully sent","email_receipt_failed":"Email receipt failed to send. Please try again.","receipt_payee":"Paid to","receipt_statement_descriptor":"This will show up on your statement as","receipt_date":"Date","receipt_transaction_id":"Transaction ID","receipt_transaction_amount":"Amount","refund_payer":"Refund from","login":"Log in to manage your payments","manage_payments":"Manage Payments","transactions_title":"Your Transactions","transaction_title":"Transaction Receipt","transaction_period":"Plan Period","arrangements_title":"Your Plans","arrangement_title":"Manage Plan","arrangement_details":"Plan Details","arrangement_id_title":"Plan ID","arrangement_payment_method_title":"Payment Method","arrangement_amount_title":"Plan Amount","arrangement_renewal_title":"Next renewal date","arrangement_action_cancel":"Cancel Plan","arrangement_action_cant_cancel":"Cancelling is currently not available.","arrangement_action_cancel_double":"Are you sure you'd like to cancel?","arrangement_cancelling":"Cancelling Plan...","arrangement_cancelled":"Plan Cancelled","arrangement_failed_to_cancel":"Failed to cancel plan","back_to_plans":"\u2190 Back to Plans","update_payment_method_verb":"Update","sca_auth_description":"Your have a pending renewal payment which requires authorization.","sca_auth_verb":"Authorize renewal payment","sca_authing_verb":"Authorizing payment","sca_authed_verb":"Payment successfully authorized!","sca_auth_failed":"Unable to authorize! Please try again.","login_button_text":"Log in","login_form_has_an_error":"Please check and fix the errors above","uppercase_search":"Search","lowercase_search":"search","uppercase_page":"Page","lowercase_page":"page","uppercase_items":"Items","lowercase_items":"items","uppercase_per":"Per","lowercase_per":"per","uppercase_of":"Of","lowercase_of":"of","back":"Back to plans","zip_code_placeholder":"Zip\/Postal Code","download_file_button_text":"Download File","input_field_instructions":{"tip_amount":{"placeholder_text":"How much would you like to tip?","initial":{"instruction_type":"normal","instruction_message":"How much would you like to tip? Choose any currency."},"empty":{"instruction_type":"error","instruction_message":"How much would you like to tip? Choose any currency."},"invalid_curency":{"instruction_type":"error","instruction_message":"Please choose a valid currency."}},"recurring":{"placeholder_text":"Recurring","initial":{"instruction_type":"normal","instruction_message":"How often would you like to give this?"},"success":{"instruction_type":"success","instruction_message":"How often would you like to give this?"},"empty":{"instruction_type":"error","instruction_message":"How often would you like to give this?"}},"name":{"placeholder_text":"Name on Credit Card","initial":{"instruction_type":"normal","instruction_message":"Enter the name on your card."},"success":{"instruction_type":"success","instruction_message":"Enter the name on your card."},"empty":{"instruction_type":"error","instruction_message":"Please enter the name on your card."}},"privacy_policy":{"terms_title":"Terms and conditions","terms_body":null,"terms_show_text":"View Terms","terms_hide_text":"Hide Terms","initial":{"instruction_type":"normal","instruction_message":"I agree to the terms."},"unchecked":{"instruction_type":"error","instruction_message":"Please agree to the terms."},"checked":{"instruction_type":"success","instruction_message":"I agree to the terms."}},"email":{"placeholder_text":"Your email address","initial":{"instruction_type":"normal","instruction_message":"Enter your email address"},"success":{"instruction_type":"success","instruction_message":"Enter your email address"},"blank":{"instruction_type":"error","instruction_message":"Enter your email address"},"not_an_email_address":{"instruction_type":"error","instruction_message":"Make sure you have entered a valid email address"}},"note_with_tip":{"placeholder_text":"Your note here...","initial":{"instruction_type":"normal","instruction_message":"Attach a note to your tip (optional)"},"empty":{"instruction_type":"normal","instruction_message":"Attach a note to your tip (optional)"},"not_empty_initial":{"instruction_type":"normal","instruction_message":"Attach a note to your tip (optional)"},"saving":{"instruction_type":"normal","instruction_message":"Saving note..."},"success":{"instruction_type":"success","instruction_message":"Note successfully saved!"},"error":{"instruction_type":"error","instruction_message":"Unable to save note note at this time. Please try again."}},"email_for_login_code":{"placeholder_text":"Your email address","initial":{"instruction_type":"normal","instruction_message":"Enter your email to log in."},"success":{"instruction_type":"success","instruction_message":"Enter your email to log in."},"blank":{"instruction_type":"error","instruction_message":"Enter your email to log in."},"empty":{"instruction_type":"error","instruction_message":"Enter your email to log in."}},"login_code":{"initial":{"instruction_type":"normal","instruction_message":"Check your email and enter the login code."},"success":{"instruction_type":"success","instruction_message":"Check your email and enter the login code."},"blank":{"instruction_type":"error","instruction_message":"Check your email and enter the login code."},"empty":{"instruction_type":"error","instruction_message":"Check your email and enter the login code."}},"stripe_all_in_one":{"initial":{"instruction_type":"normal","instruction_message":"Enter your credit card details here."},"empty":{"instruction_type":"error","instruction_message":"Enter your credit card details here."},"success":{"instruction_type":"normal","instruction_message":"Enter your credit card details here."},"invalid_number":{"instruction_type":"error","instruction_message":"The card number is not a valid credit card number."},"invalid_expiry_month":{"instruction_type":"error","instruction_message":"The card's expiration month is invalid."},"invalid_expiry_year":{"instruction_type":"error","instruction_message":"The card's expiration year is invalid."},"invalid_cvc":{"instruction_type":"error","instruction_message":"The card's security code is invalid."},"incorrect_number":{"instruction_type":"error","instruction_message":"The card number is incorrect."},"incomplete_number":{"instruction_type":"error","instruction_message":"The card number is incomplete."},"incomplete_cvc":{"instruction_type":"error","instruction_message":"The card's security code is incomplete."},"incomplete_expiry":{"instruction_type":"error","instruction_message":"The card's expiration date is incomplete."},"incomplete_zip":{"instruction_type":"error","instruction_message":"The card's zip code is incomplete."},"expired_card":{"instruction_type":"error","instruction_message":"The card has expired."},"incorrect_cvc":{"instruction_type":"error","instruction_message":"The card's security code is incorrect."},"incorrect_zip":{"instruction_type":"error","instruction_message":"The card's zip code failed validation."},"invalid_expiry_year_past":{"instruction_type":"error","instruction_message":"The card's expiration year is in the past"},"card_declined":{"instruction_type":"error","instruction_message":"The card was declined."},"missing":{"instruction_type":"error","instruction_message":"There is no card on a customer that is being charged."},"processing_error":{"instruction_type":"error","instruction_message":"An error occurred while processing the card."},"invalid_request_error":{"instruction_type":"error","instruction_message":"Unable to process this payment, please try again or use alternative method."},"invalid_sofort_country":{"instruction_type":"error","instruction_message":"The billing country is not accepted by SOFORT. Please try another country."}}}},"fetched_oembed_html":false}
One Reply to “New Sender-Recipient Limits for Exchange Online Coming in September 2021”