How to limit outgoing (SMTP) email service based on SMTP user account in postfix

Oh boy, I love these servers. Almost universally, they're doing content-based scanning of the message bodies and then sending non-200 responses to the DATA command if they don't like the contents. This is fine in principle, but when you get some under-resourced crapbox running Exchange and some overweight proprietary waste of disk space doing the scanning, the machine runs like molasses and everyone else times out.

Yes, in theory this could be an MTU problem as you've suggested, but in practice this is pretty unlikely -- if you can get a 2000 byte message through, that disproves that theory (and there aren't many messages less than 2000 bytes, including headers, out there). A tcpdump at your end can confirm this -- if it's a TCP level problem, then you'll see attempts by your end to retransmit the full-size packet; if it's a remote MTA-being-slow problem, then there won't be any retransmissions, and the stall point will be when your end sends the closing ".".

Given that the far end seems clueless ("everybody else doesn't have a problem" my prominent behind), I'd just increase Postfix's timeout to some significantly higher figure and leave things be. The three settings you want to look into are smtp_data_done_timeout, smtp_data_xfer_timeout, and smtp_data_init_timeout (in roughly that order of importance). As you can see, the defaults are mighty generous, so the need to bump them up really does indicate how bodgy the far end is.

Rephrasing what Michael has said, you need to enable smtp authentication for this to start working. If you do not enable smtp authentication then spammer's will start using your server to send out emails. You can use Postfix + Cyrus SASL + Mysql as that mentioned in the below URL to set it up.

http://flurdy.com/docs/postfix/edition5.html#conf_auth

Once this is done you can make your one web application which gives your customers to reset their password and all that value add stuff.