Emails from t-online to freenet disappear without a trace?

A friend's emails haven't arrived for a few days. They were sent without error for him. However, there's no sign of them in my spam/virus folder. Not even after waiting two days. We tested it and found that it depends on the provider:

• konto1@t-online.de -> @freenet.de: fail
• konto1@t-online.de -> @gmx.de: OK
• @gmx.de -> @freenet.de: OK
• konto2@t-online.de -> @freenet.de: fail
• konto2@t-online.de -> @gmx.de: OK

Ergo: ONLY the emails from t-online to freenet disappear

(account1/account2 are different email accounts and not just alias names for the same account)

This is 100% reproducible, regardless of the mail content and regardless of the client (Thunderbird or via browser).

Has anyone else noticed this? Can anyone reproduce it (t-online -> freenet)?