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Remote Server returned '550 5.6.0 CAT.InvalidContent.Exception: ExchangeDataException, Decoding of header Subject failed+ADs- raw value: +AFs-Ps+AF0- +AD0-?iso8859-7?b?wer98fnz5yDF8OX05enh6t7yIOXq5N7r+-fPn8iDz9Oft?+AD0-; cannot handle content of message

It looks like the Subject: header of the message is in Greek and is RFC2047 encoded in the iso-8859-7 character set.

For some reason, Hotmail doesn't accept this.

...

That happened since September 10th this year. Before, all received without problem.

This is a Hotmail (Microsoft) issue. I don't know if communicating with them would help or not.

You could change Mailman's encoding for Greek from iso-8859-7 to utf-8, but this is an involved process. You have to recode Mailman's messages/el/LC_MESSAGES/mailman.po and templates/el/* and any custom templates from iso-8859-7 to utf-8, run msgfmt to recompile messages/el/LC_MESSAGES/mailman.mo and also recode any Greek strings in the list configurations. There is a script at https://www.msapiro.net/scripts/recode_list (mirrored at http://fog.ccsf.edu/~msapiro/scripts/recode_list) that can help with this last step.

I also don't know that changing the encoding from iso-8859-7 to utf-8 will help with this Hotmail issue or not, but it might be worth a try.

OK, I'll try it. Thank you for your answer. Nikos.