When trying to access web mail I receive error 405 method not allowed

Read this; and this... or just skip to this:

Assuming that your web-based email interface is running on a Windows IIS web server, then the most likely problem is that said server is misconfigured so that the POST method of form usage isn't working.

EDIT: Or, I just remembered... if the web site that's running the web-based email interface is using "cPanel" as its hosting control panel (regardless what flavor of web server on which it's running), then there is a known error in cPanel that screws with the MX record in the DNS zone file (an internal thing that the webmaster of the site hosting the webmail inteface should know about) that can cause 405 errors during the POST process.

Contact whomever owns/runs the web site and describe, in vivid detail, precisely what you were doing in the moments before you got the error; exactly what it was that actually triggered said error; and all the details of what kind of computer, operating system, and web browser you're using.