Thunderbird on my computers (like Timmay I notice it's getting slower and slower), K9 on Android.
I have a few different email addresses for various reasons so like the fact these apps can show unified views. Emails that I want to keep get moved, via Thunderbird, to a NAS drive - what I'd really like on my Android is an email client that could do the same (including the ability to read already stored emails) but I'm not gonna hold my breath waiting for one.

