This is somewhat redundant now, but the information is a little more precise than previoius replies.
Sascha Rissel writes:
again I'm getting bounces for one specific provider: Germany's T-Online.
[...]
<some_user@t-online.de>: host mx03.t-online.de[194.25.134.73] refused to talk to me: 554 IP=62.138.1.209 - A problem occurred. (Ask your postmaster for help or to contact tosa@rx.t-online.de to clarify.) (BL)
At a guess "TOSA" = "terms of service agreement" and "BL" = "block list". That's supported by the SMTP status 554 (denied for administrative reasons) and the link below. But http://mxtoolbox.com/ says you're not on any of the 95 RBLs it checks. So it must be internal. Google is my friend, and told me to look at
http://serverfault.com/questions/602399/t-online-de-blacklist-host-refused-t...
So it looks like t-online has taken a severe dislike to you ("BL" apparently means they've decided you're a long-term bad actor). If you are in the fortunate position I am[1][2], you tell the user that t-online is f++ked and they should get a new address. If you're not, write to tosa@rx.t-online.de and beg to be reinstated in their good graces.
Footnotes: [1] You can generally ignore my footnotes. It's my way of denoting a a-political rant. :-)
[2] My employer, the Japanese Ministry of Education, prohibited use of Yahoo addresses for academic business after the Yahoo!/AOL April Fool's Joke of 2014. But strictly speaking, it turns out that it's the Ministry that's f++ked because they can't tell the difference between yahoo.co.jp, which causes no DMARC problem (they don't have a p=reject policy), and yahoo.com, which has both DMARC *and* privacy issues out the wazoo. But I ignore that, since I hate the garbage that Yahoo's wannabe MUA emits. ;-)