My company has two domains: company.com and company.be, and have e-mail accounts on both. When sending from a .com address to a .com address, or from .be to a .com address, all is right. But when an e-mail is sent from a .com address to a .be address, the e-mail arrives at the .com account (in the case where both exist, so employee_a@company.be and employee_a@company.com both exist and are separate accounts the the thunderbird client, but are used by the same employee). Strange thing is that this does not happen when the e-mail is sent from a mailclient on a mobile phone that is configured in the same way) I've been searching a long time and can't seem to find a solution. MX records for company.com is set to mail.company.com which is the IMAP server.

  • What are the MX records for example.be? If they point to mail.example.com that would be an indicator of why. Even if the MX records are correct, if you are sending via mail.example.com and mail.example.com is configured to handle mail for both example.com and example.be then that would also be a reason why. – ivanivan Apr 5 '18 at 16:31
  • Thanks for the response. MX records for company.be point to mx1, 2 ... .ovh.net (.be domain is registered at OVH) and MX records for company.com point to mail.company.com. – Arno Apr 5 '18 at 16:37
  • Does this only exist on the local data network internally though from the Thunderbird client? Can you try doing this from a newly configured Thunderbird client outside the data network via IMAP or whatever and see if the result is different on the outside of the network? Perhaps there's some statically cached DNS record pointer or something causing this per DNS resolution internally on the incoming and outgoing side of the client configuration. Specifically the outgoing and incoming Server Name in the TB configuration – Pimp Juice IT Apr 5 '18 at 16:41
  • I'm not sure what internal DNS points to internally for email server connections but if you have it pointed internally different than externally, this is something to check just in case. While the MX records are correct from the outside world, just double check it's directing to the correct internal servers, etc. internally in case that's what is going on here—simple enough to eliminate as a cause at least. I assume it's likely be the outgoing Server Name but I'd check them all, internal DNS resolution, etc. – Pimp Juice IT Apr 5 '18 at 16:43
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    @PimpJuiceIT Likewise – davidgo Apr 5 '18 at 21:44

This issue turned out to be a Synolgy DSM MailServer settings problem. The setting 'Hostname (FQDN)' was set to mail.company.com, when it should have been company.com and 'additonal domains' also listed company.be and company.com. Removing the additional domains and changing the FQDN solved the problem. Thanks for the help

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