I have two email stuck in outbox since three days. Why do...

This is not unique with Tuhund. Email in ERP work the same way they work in email clients except that in ERP email is tightly integrated with rest of the system and comes with more features.

When an email address of any recipient in the email is wrong, email bounces back with failure notice. This happens because your email server accepts the email message and attempts to relay it to the recipients' email server. It is the recipient's email server that returns an error message. Exact text of the failure notice depends on the email server.  If you are sending email to multiple recipients, only the ones with wrong email address fail and you get separate failure notice for each email address.

However, if the domain of the wrong email address is same as the domain of the sender. Which means if email of wrong email address is hosted on the same email server as yours, message fails to go out. It does not reach any recipient even those with correct email address. In such a case, you just need to remove the wrong email address and te message will go out.

We usually do not notice this error as it is very rare. You usually do not have the email address of your co-worker wrong.