Prasanth Nair writes:
*2%Failed adding message <mailman.2.1593699619.32248>: 'ascii' codec can't encode character '\ufffd' in position 252: ordinal not in range(128).*
This is the Unicode REPLACEMENT CHARACTER, which means that something along the line, possibly Mailman, could not decode some bytes, and replaced them with that character. The message about "ASCII" codec usually means that this occurred in the header of the email, which is typical of spam. The message ID was generated by Mailman not by the originating system, which is also indicative of spam.
If it is spam, you can identify the message and delete it from the archive. If it's not, you will need to find the text that is being replaced and delete it or replace it with ASCII yourself.
I ran the check_hk_import script before migrating and it was not giving any error.
In order to catch this kind of error, the check script would have to pretty much do the import but throw it away.