While sending emails from your email address and you get the error Unrouteable address with RCPT reject. That means the problem at receivers end.
Check the following things.
1. Check the domains entry is available in userdoamins.
2. Check the domains entry is is available in localdomain
3. check the domain must having catchall address set if not then set
4. also make sure that the mail folder having ownership user:mail
5. restart exim services once.