I have to say that Thunderbird beats the others hands down if only because of the superb way it handles its database. Backups are simple and restoration easy too. My only experience of Kmail backing up was horribly drawn out because it keeps each email as a separate file. I have virtually all my emails since 1996 and that's an awful lot of files in Kmail. There is also a major deficiency in all other Linux email progammes that I have tried. There is no quick way to delete an attachment but keep the email. Thunderbird is the only one that does that.
Self-righteousness is a loud din raised to drown the voice of guilt within us - Eric Hoffer
Don't believe what it says on the right - I am anything but sagely; More oniony!