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Do you have a recommendation for a good GUI mail client?

Claws mail is pretty good IMO.

There are some more options here:

Thanks, last time I tested aerc I couldn't find a way to make it download all my mails locally. Is this possible?

No, but you can use it in combination with something like imapsync.

Try mbsync or OfflineIMAP. Both great tools to sync a remote mailbox to local.