Use "fetchmail".
Fetchmail will collect mail from any email account you have, POP3 or IMAP, on any other ISP or domain hosting. It will deliver that mail to the account you nominate. It can do this for hundreds of user accounts, each with many email addresses on the internet.
In your situation, you have one local user, with several external email addresses. Install "fetchmail" and configure it to collect all your external emails, which it will then deliver to your internal mailbox.
The author provides sufficient documentation for you to sort it out.
Best advice for sending mail is to do it through your ISP SMTP server, not through your SME SMTP.....some ISP's don't like being used as a "mail relay". (BUT - if they don't mind, then use the SME SMTP server.)
Here is a link
http://wiki.contribs.org/FetchmailCheers
Ian