Universal mail client?

Question

Email

Hello.
The question may be obscenely banal, but still I did not find a solution to it.
There are several email addresses in different locations. From the axes I use Win7-8, android and, sometimes, debian. We need a universal, cross-platform solution for working with all postal addresses (as an option, a web service).
Necessarily, what is needed is the synchronization of accounts (as an option, I can store any file with data in the dropbox), so as not to enter everything again each time. Only completely free (I can buy it at home, but at work I don’t need to buy software once again). Naturally, we need not only the collection of letters, but also the ability to send letters from the address I need.

Answer 1

Yandex or gmail

Answer 2

any email client that can imap, no?

Answer 3

Thunderbird on desktop, K-9 Mail on Android
Both Open Source