Whitelist our mobile phone users

[Nigel Kendrick]

We have a recurring problem (2-3 times a year) where UK Vodaphone domains
get added to various RBLs and so our mobile users suddenly get their mail
rejected as outbound spam by our mail server (MailScanner, Spamassassin and
Postfix). Considering these users have to do an SMTP login to send mail, is
it possible to whitelist them based on this fact? If not, what's the best
way to cope with this without letting too much spam through - or do I just
not scan outbound mail!?

Thanks,

Nigel Kendrick