Invalid input to moderatorgroups results in malformed SMTP

[cathoderaydude]

I'll preface this by saying this is user error through and through, but since the failure mode results in exposing SMTP server traffic to end users, it might make sense for the plugin to handle it.

I spent several hours trying to figure out why I was getting "554 5.5.1 Error: no valid recipients" back from my mail server every time anyone posted a comment. I confirmed in the log that it was genuinely not sending any recipients to the server, but couldn't figure out why. I found that if I cleared the "moderatorgroups" field it went away. I then realized I had put a group name in the field without a leading @, causing it to interpret it as a user, which did not exist.

I found that when discussion creates the list of bccs, it does not check whether the list of moderator users is nonzero first. If no valid users or groups were found, it proceeds to iterate over an empty set, producing an empty string, then tries to Bcc it. I patched this in mine and will see if I can create a PR; I am not great at github.