Emails with sender MISSING_MAILBOX@missing_domain

Hello,

we are working in the migration from courier-imap to dovecot 2.2.10-4.el7_0.1.x86_64.

Some emails that send a internal application show as sender "MISSING_MAILBOX@missing_domain", i use like email client Horde using dovecot like server. But if i open that email with Horde and courier-imap i can see the correct email sender.

I think that the email of the sender is no correct or not accomplish some RFC. Can anyone help me to understand this?

Thank you