I'm a spammer!

[#1]

I have been communicating with a friend in Germany for some while. He's recently changed to GMX as his mail provider, and has just reported that  "For some reason your mails to this address always land in my suspected spam folder??".  I'm using my genuine ***.plus.com email address for both my From: and Reply-To: addresses, but I am wondering whether the fact that the Plus mail servers are .plus.net is being seen as an inconsistency and thus causing his ISP to flag me as a spammer?
Does anyone know, or have a better theory?

[#2]

I can never remember whether it's username@plusdsl.com or username@plusdsl.net when it comes to setting up a router.  It would be so nice if Plusnet could use .plus.com for its mail servers then I could forget about the .net option completely.  Can't this be done using some sort of aliasing (if that is the right term) so that relay/mail.plus.net and relay/mail.plus.com resolve to the same computer?

[#3]

My email addresses are domain related and have no connection at all with the relay names. This is not unusual.
I doubt that is the cause - more likely something related to the way they check for spam. If information in the email headers doesn't give any clues I think your friend needs to check with GMX support.

[#4]

Quote from: spraxyt

If information in the email headers doesn't give any clues I think your friend needs to check with GMX support.

Agreed.

[#5]

Thanks everyone and yes, clearly the ball is in GMX's court, and I suspect over-enthusiastic settings in their spam-trap. My friend isn't particularly technically literate, so even getting him to copy and paste the headers might be a problem   I'll try to steer him through the 'find the headers' and see if he can copy and paste them back to me.