I have a working exim setup with virtual users, working alright, except for when I try to send email to certain servers. These servers reject my emails because of #5.1.3 Invalid character ('@') in username.

The offending header parts seem to be:

Return-path: <"[email protected]"@smtp.example.com>

and

...(envelope-from <"[email protected]"@smtp.example.com>)...

The problem is that I cannot find where and why the usernames are being generated like this.

My router for submission is:

dnslookup:
  driver = dnslookup
  domains = ! +local_domains
  transport = remote_smtp
  ignore_target_hosts = 0.0.0.0 : 127.0.0.0/8
  no_more

And the respective transport:

remote_smtp:
  driver = smtp

What can be producing this problem?