Emails are not being received?

I'm currently having a problem where certain emails aren't being delivered. However, this only seems to happen when I enter my email address somewhere. Normal email communication with colleagues, etc., works fine, but when I have to enter my email address, for example, to sign up for a newsletter, set up an account, or something else, the emails I'm supposed to receive for confirmations, verification, etc., never arrive. This has happened to me five different times in the last few weeks, and I'm starting to get frustrated.
If it matters, I use T-Online and generally check my emails on Apple devices.