Hi,
as far as I know, there are two solutions :
1) Use the SMTP of your provider as smart host for your SME server. No change on your computer(s) on your LAN, but SME send all mail to your provider SMTP server (witch should be allowed to send mail to big mail-provider

) . This change nothing for incoming mails, so all antivir solution should be OK.
2) Use externals mailboxes, but use the (new

) Fetchmail contrib to retrieve externals mail on your SME server. Your antivir solution should also be OK.
HTH,
Pascal