This morning I pushed out beta-4 for Email Users v.4.5.3. This latest update addresses the duplicate header information discovered by Andy Fragen. I had already fixed the problem with the TO header Andy noted but was unaware of the duplicate MIME-Version header. It turns out, there was also a duplicate X-Mailer header as well.
By default wp_mail() sits on top of PHPMailer. WordPress does not check for either the MIME-Version header nor the X-Mailer header. Unless specifically set, PHPMailer will add its default values for these two headers which is what was happening. It appears that some systems are sensitive to duplicate headers and reject the messages and others do not. This definitely explains why it is difficult to reproduce reports that come in of users not receiving email.
Because wp_mail() is pluggable, other mail implementations may be expecting these headers to continue to exist. What I have done is added two new options to add the headers as Email Users has been doing all along. By default these two options are off because PHPMailer duplicates them and I expect most people use the default WordPress implementation.
I also improved the information panel I added recently to report the existance of any filters which may interact with Email Users. For the most part these filters don’t do any harm however they can change override the FROM information in the email or the format which may be unexpected. The new information in the panel is nothing more than a note that something else may be affecting your email outside of Email Users (e.g. why is my From name different than what I set?).
Please report any issues with this latest beta update.