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Use a real domain name that is valid on your sme server, and resolves from the Internet and from your LAN, and DO NOT use a domain name that ends with .local
I suggest you use the Fetchmail contrib which is known to work satisfactorily with sme server see
http://wiki.contribs.org/Fetchmail Further to this I strongly suggest you ditch the idea of configuring your sme server to collect mail from your ISP's POP account, and instead use domain based email on your server (ie smtp mail server). SME is already configured by default in this mode, and you can use a free domain name or register a new one that is more appropriate.
Then forward mail from your POP account to a user@domain on the sme server. This is a much more reliable solution.
Then eventually advertise the new domain based email address rather than the POP account address.