I know this is an old thread, but I wanted to post, because it looks like the original issue was never resolved. I, too, had been using GMX for email notifications through my M1 Gold's XEP. It had been working fine for years until July 2017. Then, all of a sudden, I stopped getting email notifications. I called my ISP, who insisted it was not a problem on their end. After months of research and many attempts to get something to work, I discovered that GMX changed something with their emails last July. I don't know all the technical jargon. However, I created an account with SMTP2GO.com, and once again, I am receiving email notifications through XEP. One think to note, however, is that SMTP2GO.com recommends port 2525, which did not work with my FiOS by Frontier router, but port 587 does work.
