Handling spam on the road with a laptop, an iPhone, and a cloud-based IMAP server

I currently run my own mail server on linode, and I use client-side spam filtering on my Mac.  It works great.  If I happen to get spam in my inbox, which happens maybe once or twice a week, I mark it as spam using a handy GUI in my email client and the email is moved to the spam filter, and the filter learns from the email.  Similarly, if a message goes to spam erroneously, I simply mark it as “not spam”, the message is moved to my inbox, and the filter corrects itself again.

This system works because I have three main machines:  a desktop, a laptop, and my iPhone.  The desktop runs my “main client,” so I leave it always on and always filtering spam.  In this way, when I check email on my phone or laptop, the spam has been filtered.

I’m not going to have an always-on machine as I transition to a mobile lifestyle.  So I have been looking at server-side spam filtering.  The trouble with that is the interface.  How do I mark an email as spam or not spam?  The process of training on a server is much more cumbersome and spammers continually adapt to the filters.  I wonder, is there a reasonably good server-side spam filter system out there for my circumstances?