I once had a computer running all the time to run POPFile and filter my Gmail messages through IMAP.

At that time POPFile+IMAP was not very stable and POPFile would crash often. And also, it was a pain to have a computer running all the time at home only for this.

Then, there was another problem. POPFile would run every two or five minutes, so when I opened Gmail I used to find not yet filtered mails in the inbox.

If you make a webservice that does this right, I will consider using it. Now I'm using Opera Mail, which is very nice, but the training is local and, therefore, I don't have the same filters and training at home and at the office, which is rather painful, sometimes.

I'm sure there is a succesfull business behind this.

Well, I probably wouldn't use POPFile itself for this anyway because I have a blindingly fast C version of the same classification techniques called polymail which I license to people. Would just have to graft on the IMAP part. Hmm.

Evening project?

Clearly, the not yet filtered bit is a nuisance, that's why you really want it to be built into your mail client.

The good thing is that now Gmail's IMAP supports OAuth, so people migh be much less reluctant to give you access to their mailboxes.

Yes, the nuisance is there, but I'm sure there is a way to solve it. The good part is that it does not make the service unusable, so, people can start using the service even though with this nuisance.

Release early and then iterate. ;-)