Only you can decide if email is important to have delivered to your phone.

Domain name is the part of the email address to the right of the "@". So, an email address of something@gmail.com has a domain of gmail.com.

If you can choose one or the other, I'd choose IMAP over POP3 for mail delivery. POP3 and IMAP are both protocols for receiving mail from a mail server, but POP3 does not sync read and unread messages with a collection of mail from a server (usually); it is designed to download all of your mail and work on the mail on your mail client. IMAP stores messages on the mail server by design and synchronizes whether you have read messages, etc. with other mail clients. However, not all mail servers support IMAP.

Your email provider will need to provide the details for connecting to an IMAP or POP3 mail server (as well as the server used for mail delivery, which uses a protocol called SMTP.)

By the way, if you have an Eris, you should use a third-party mail client for Android such as K9 mail rather than the stock email application, which has a serious bug that affects phone performance.