A way to catching incoming emails

Question

I am looking into how I can develop an ability to send information into the system via email.

Twitter has a feature with mobile phones, your mobile is registered with their system and when text comes in from your phone they would post a tweet..

I am looking to have a similar feature with emails, where by user A has email userA@domain.com and when we have an incoming email we will know how to deal with...

At the present moment I have following solution:

1. Create an email box for each account, have it automatically created for each new account
2. use fMailbox to catch new emails, using cronjob every 5 minutes or so.

My thinking is that if you have 1-3000 users it would be quite a lot of emails to check?

In perfect scenario I would want to:

1. Look at all email incoming emails that have date older or equal to the previous check
2. Filter out emails with emails that we have registered in the database
3. Process emails
4. Delete emails

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Extra condition is to make sure that

1. Email is not a spoof
2. Email is not malicios