I have a Linux CENTOS Server running PostFix. Many of our users 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 Server and are deferred with messages like:

(conversation with alt1.gmail-smtp-in.l.google.com[173.194.72.27] timed out while sending message body)

Most of these emails have attachments, so probably they are a around 2 or 3 MB. We realized that small emails pass and are received normally.

In this regard any advice? It will be appreciate.

Have you experimented with sending emails directly from PostFix rather than via a third part SMTP server?

I did this many years ago to send emails without any size limit. Clearly only works if the receiving client is happy with this.

Some ISPs may only process such emails from fixed IPs for spam reasons.

I am not saying this is necessarily a good permanent solution but it may be useful for testing purposes.

If you do delve into PostFix make sure you know how to reverse it or use a dedicated test environment.