In short, there are a lots of reasons why you get "550-Verification failed for".

One reason for this behavior is that your domain is banned on the MTA server. Maybe your MTA does reverse DNS checks and it turns out you're host.foo.bar and try to send messages as tuxie@goro.foo

Some MTA's do checks for Sender's validity (the existence of that user) in case they relay mails for you.

etc. etc.

Either you're not using well the MTA, or it isn't properly configured.