Email not getting delivered

[Paul Klos]

> Just in case it's relevant, my /var/lib/imap/socket/lmtp has the same
> permissions (srwxrwxrwx)
> 
Same here, so that in itself can't be the issue. You might try adding a 
-v for more verbosity in the relevant entries in Postfix' master.cf, 
which will hopefully provide something useful.

Is there any possibility that these mails are rejected based on policy, 
for example because of an invalid sender?

Paul