How to migrate Mail from EOS 5 to 6

[frank-wo]

i know this is not an issue but a question ( i have no idea how to get in contact with EOS support):

Since Mail is broken in EOS 5 (#677) I'm looking for a solution to get my message base and email addresses back.

My first question is: is it possible to upgrade the old Mail app to the new one, but on EOS 5.1.7 - without installing EOS 6.

My second question: where all email data has been saved on Mail in EOS 5.1.7., how can I save this data when migrating to EOS 6.

TIA
Frank