XF 1.5 Many users fail to receive confirmation email

Many users report not receiving an email confirmation. It looks like most of the users are from Germany with a web.de or .gmx.net email address. We get email bounces about them.

Can you help me understand if this is an issue with Xenforo or our servers and what you might recommend to resolve this? I'm guessing others have had this issue even if it's not an issue with Xenforo.

Thank you,