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Attempting to deliver more than a few hundred e-mails per day via SMTP to our own employees on Google G-Suite tends to cause significant delays due to arbitrary quota limitations on the Google side.

We currently have a single corporate postfix server responsible for outgoing SMTP. This server relays any e-mail from internal services to internal recipients to G-Suite.

I have SFP records configured and the e-mail server is registered within G-Suite as a company e-mail gateway.

Often e-mail delivery to G-Suite is rejected with a 450-4.2.1 response from the Google mail exchangers, causing hours of delay.

Specifically this message:

Jan 10 11:57:03 smtp postfix/smtp[7127]: A067338013: host aspmx.l.google.com[173.194.73.26] said: 450-4.2.1 The user you are trying to contact is receiving mail at a rate that 450-4.2.1 prevents additional messages from being delivered. Please resend your 450-4.2.1 message at a later time. If the user is able to receive mail at that 450-4.2.1 time, your message will be delivered. For more information, please 450-4.2.1 visit 450 4.2.1 https://support.google.com/mail/?p=ReceivingRate r22-v6si64987734ljb.4 - gsmtp (in reply to RCPT TO command)

This illustration shows the current flow of e-mail from internal systems:

Current flow of e-mail from internal systems to G-Suite

What options may be available to increase the capacity for e-mail from internal systems to employees?

Might we have overlooked some configuration option in G-Suite? Is there some other way to deliver e-mail to G-Suite from internal sources? Can we add more SMTP relays to increase the quotas?

I have discussed the matter with G-Suite Support, who's only recommendation was to send less e-mail. Go figure :)

Considering we are only sending a couple of dozen e-mails per user per day, I'm guessing we are not the first company to see this issue. Hopefully there is a simple solution!

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It looks like Google offers a G-Suite SMTP relay service for outbound e-mail that may also be used to transfer a higher volume of e-mail to internal recipients, hopefully bypassing the per user receive limits that are applied to the Google public mail exchangers (these public mail servers are referred to as Restricted SMTP Service by Google).

A specific relay service must be configured in the advanced Gmail settings in the G-Suite Admin interface. Then a local SMTP server can be added by IP address or IP range. Finally, the local SMTP server must be configured to deliver e-mail for your domains directly to smtp-relay.gmail.com instead of using the MX records in DNS.

An organization wide quota will then be in effect, limiting the number of e-mails per day to 130 multiplied by the number of G-Suite accounts. So for a 50 user organization that is 6500 e-mails per day.

It is unclear whether other short term quotas are also in effect, similar to those enforced on the Google public mail exchangers.

The different e-mail delivery services are described by Google here: Send email from a printer, scanner, or app

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