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 email@example.com and firstname.lastname@example.org 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.
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