If you don’t use Gmail as you regular device email client, K9-Mail is pretty much the best third party email client available for Android. There is no one standout feature of K9 that makes it better than other alternatives but collectively it does all the small things that are important better.
The App supports adding multiple email accounts in the usual manner through pop3 or imap. Some of the more common email accounts will pretty much set themselves up once you enter an email address and password but others will require you to enter settings manually to complete the setup. It is worth having these settings to hand before you start adding email accounts. The settings can easily be obtained through a simple Google search but if you are having any problems post a comment below and we’ll help you out.
Different accounts are given different colour codings to differentiate them form each other. This is helpful in the unified inbox feature that is available where all email accounts inbox contents are collected into one so you don’t need to go in and out of each account to read your new emails.
The App icon itself also has a handy indicator of how many new emails you have in the combined accounts.
If you are looking for an email client other than Gmail, K9 will be a good place to start.