linux - Messages deferred (time out while sending message body)

I have a Linux CENTOS Server running PostFix. It has no mailboxes, but is a mail gateway, for several Domains in another destination Server, which is also a Linux CENTOS running Sendmail. So, after checking the emails with antivirus and antispam, the Postfix Server delivers them to the Sendmail Server which hosts the real mailboxes.

Many of our customers are claiming they don't receive several emails, or receive them with many hours delay, so we monitored the /var/log/maillog file at the Postfix Server, and found out that those emails are not being delivered to the Sendmail Server and are deferred with messages like:

Aug 23 11:48:58 srv7 postfix/smtp[618]: 980C773D64B: to=, relay=srv6.multisitesdominios.com.br[200.184.161.136], delay=2375, status=deferred (conversation with srv6.multisitesdominios.com.br[200.184.161.136] timed out while sending message body)

Most of these emails have attachments, so probably they are a little large. We realized that small emails pass and are received normally.

My questions:

1. What is the real reason of this problem, is it really the emails size?


2. Is there any Postfix parameter we should tune to avoid it?


3. Would the problem be at the destination Server (the Sendmail one) and not at this "gateway" Server (the Postfix one)?


4. What would be the definite solution?

Answer

1. It's indirectly the size. It takes too long without receiving progress on the data so that either side hangs up



2. smtp_data_xfer_timeout. It defaults to 180 seconds which should be OK in every scenario.


3. It can be on both. But probably it is the network in between.


4. Check why the communication stalls.