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Hi, that is not a problem with the plugin, is the receiving server refusing emails from your server, you should check with your provider, especially if that error happens with all emails. In this latter case, the problem can be in the mailing service of your hosting provider.
Well, it *is* a problem with the plugin, as long as you do not emphasize that a separate SMTP plugin is definitely needed to meet all the antispam regulations of email providers.
In fact, there are plugins around that include a HELO/EHLO command by default (which is not very difficult). I thought you were able to do so, too.
Anyway, I downloaded one of the SMTP plugins around, and I hope this will work.