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I am currently trying to get my head into learning how Linux systems send mail, starting from Apache, to Sendmail and finally out the door.

Specifically I'm using CentOS 5 which is using Sendmail, I'm fairly sure it's the vanilla setup although I didn't set it up myself.

I'm interested in things like, how mail is queued when a service like sendmail is disabled, how Apache and Sendmail talk to each other, what other options are there instead of Sendmail etc.

My most immediate mission is to figure out how to create a whitelist for sending out, so that mail can only be sent to specific recipients, and if a mail is blocked then it's either rerouted to a specified address. I'm in no way partial to sendmail, and if performing such a task involved switching software then I'm all for it.

The server itself is a staging server, however it recently managed to send out some production data to production email addresses which caused a bit of embarrassment and so I've been tasked with locking it down, however I'm pretty fresh to the entire mailing process on Linux.

Edit: Additionally, the current state of the server I'm working on is that; sendmail is disabled and a bunch of emails are now stacked up in clientmqueue, however some of these don't appear to hold any recipient data. Is recipient data held elsewhere? What is likely to happen if I now enable sendmail, will these items send out?

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Perhaps this overview helps...

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While this link may answer the question, it is better to include the essential parts of the answer here and provide the link for reference. Link-only answers can become invalid if the linked page changes. –  Shadur Jan 25 '13 at 5:48
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