Code review comment for lp:~bhavesh-goyal093/postorius/fixed-invalid-email-acceptance

I *think* that invalid email addresses are probably rejected by Mailman Core anyhow, but this should let us provide better error messages. I'm running some tests against the code now in preparation for merging.

If someone could set up a few tox tests for this so it'll be tested going forwards, that would be most excellent.