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I'm trying to fix a problem with a slow line: whenever a user sends a message to be relayed through the company's postfix, as soon as the SMTP conversation finishes, postfix immediately queues the message toward the smtp transport.

When the user is inside the LAN, all is well; when the user is coming through the WAN, not so much.

Our users use IMAP accounts which are served by the same ADSL line that serves the postfix server, so when the MUA copies the message to the Sent folder, it finds the line clogged by the outgoing message.

In sendmail, there's a "HoldExpensive" option that causes the server to skip the first delivery attempt, waiting for the next queue run instead.

In postfix, I found the defer_transport option, but it seems to put the messages in an indefinite hold.

How can I emulate the "HoldExpensive" behaviour with postfix?

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As far as I can tell, there is no such feature in Postfix.

You could use an access policy with the HOLD target that puts the message on hold when it is coming from a WAN user. Afterwards, you could use postsuper from a cron job to release the held messages when it's convenient. The drawback of this would be that it also delays mail to local recipients that wouldn't need to be transported away via SMTP.

The more efficient solution might probably be to just give the end users some guaranteed IMAP bandwidth by enabling traffic shaping on the firewall, or even on the mail server itself.

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