Issue Receiving Emails from Microsoft 365 to Multiple Recipients on cPanel Server

Abdelrahman Mohamed

• July 24, 2025 20:02

Hello, I’m facing an issue where emails sent from a Microsoft 365 account to multiple recipients (hosted on my cPanel mail server) fail to deliver, but only when the recipient count exceeds a certain number (e.g., 17 recipients). Here's what I’ve observed: Sending from Gmail to the same cPanel recipients works fine, even with 20+ recipients. Sending from Microsoft 365 to 12 recipients works fine. Sending from Microsoft 365 to 17+ recipients fails, emails remain stuck in “pending” in Microsoft 365 and never reach my server. They do not show up at all in Track Delivery in WHM or cPanel. On Microsoft 365 that's the feedback: returned '550 5.4.300 Message expired -> 451 4.4.400 Error communicating with frontend host or destination host. -> 421 4.4.2 Connection dropped due to ConnectionReset'

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  cPRex Jurassic Moderator

  • July 24, 2025 22:53

  Hey there!  It's interesting that they don't show up in the Track Delivery tool.  Do you see the email when you check the exim log (/var/log/exim_mainlog) directly on the server?

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  Abdelrahman Mohamed

  • July 25, 2025 07:33

  Thats what I get in the exim_mainlog and don't see in the track delivery,

  SSL_write:..... syscall: Connection reset by peer
  SSL_write:...... syscall: Broken pipe
  TLS error (SSL_read): unexpected EOF while reading
  SMTP connection ... lost while reading message data (header)

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  cPRex Jurassic Moderator

  • July 25, 2025 14:50

  If that's the only entry in the Exim log for that file, then that tells me something is happening to the message outside of your server.  It might be best to try and contact Microsoft about this issue directly to see if they have more details on what is happening.

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