Many packages exist for reading mail in Emacs. Even out of the box Emacs comes with several competing solutions. These are all well tested and have a large user base, which certainly is a major advantage if search for documentation or need help.
Note that several of the mail packages use incompatible mailbox storage formats. While there are programs for converting between formats, it is best you already take your time to consider the supported formats when choosing your mail client.
The packages that come with Emacs are:
Some other packages that are distributed separately are also commonly used:
Some people also like to use a non-elisp mail client from within Emacs:
And then there are some less commonly used and/or badly maintained packages: