Once in a while randomly I get "501 5.5.4 Invalid Address" error for e mails sent to a puticular address. It does not happend always so it hard to trace it and solve it.
Best explanation I can find on internet is from the post WhatsUp Gold - Error 501 5.5.4. in mail test
But still I am unable to reproduce the error and check what is acctually wrong. I will update.
