Bounced e-mails sent to Gmail
During the past couple of months numerous users sending e-mail from Microsoft 365 accounts have reported when sending bulk e-mails, such as to 10-100 recipients, every Gmail address bounces. Sending to a single recipient does not seem to be an issue. In each case these are established recipient groups which have been used for several years without a problem. Though the solution is the same for all, the bounce messages differ quite a bit.
- Remote server returned ‘550 5.4.300 Message expired -> 421 4.7.28 [2a01:111:f400:****::***15] Our system has detected an unusual rate of unsolicited mail originating from your IP address. To protect our users from spam, mail sent from your IP address has been temporarily rate limited.
- Your message couldn’t be delivered. Despite repeated attempts to contact the recipient’s email system it didn’t respond.
- Your message was rejected by gmail-smtp-in.l.google.com for the following reason:
- 5.7.26 The MAIL FROM domain [xyz.com] has an SPF record with a hard fail
- 5.7.26 policy (-all) but it fails to pass SPF checks with the ip:
- 5.7.26 [209.71.208.25 Note: not the sending address]. To best protect our users from spam and phishing,
- 5.7.26 the message has been blocked.
During the past week Microsoft has posted the following alerts:
All of the clients with delivery issues have always had tested SPF records, but it seems adding DKIM and DMARC DNS records, resolved the problem in each case.
The following are some links to assist you with creating and testing your DKIM and DMARC records:
How to configure DKIM with Office 365:
How to Configure DKIM for Office 365 — LazyAdmin
How to use DKIM for email in your custom domain | Microsoft Learn
Test your DKIM entries:
Home – Microsoft 365 admin center
Network Tools: DNS,IP,Email (mxtoolbox.com)
How to configure DMARC with Office 365:
How to Configure Office 365 DMARC — LazyAdmin
LAN-Tech Network Management 