Let's suppose I have created an IMAP client and I want to create a filter that, say, deletes messages that have certain characteristics.
I can make this a startup task that happens whenever the client is opened, but the problen is that a user might view the inbox in a different client and the filter won't get rid of the unwanted messages before the inbox is viewed.
Is there some way to tell the server to call my service every time it receives a message so the unwanted messages can go away once the server receives them instead of once a client is opened? Is this even possible with IMAP? If not, is there a different well supported e-mail technology that allows this sort of interaction?
You can do server-side email filtering with the Sieve Email Filtering Language and the ManageSieve Protocol. There is good support in open-source software. However, I am only aware of a few major email providers, that support it. If it is supported, it is usually accessible on the IMAP host and with the IMAP account credentials, i.e. you can test it by connecting to TCP port 2000 and 4190.
See the following three links for more information:
Sieve also supports a few notifications types. If the filtering language is not powerful enough to for your purposes, you can send notifications for all incoming emails, wake up the IMAP client, and let the client to the filtering.