Feature: Support Amazon SES' throttled SMTP gateway by default

[cmcaine]

Is your feature request related to a problem? Please describe.

tl;dr: an email campaign I sent was not delivered to around half the contacts because listmonk didn't understand amazon SES' throttling messages.

Longer: It's difficult to send mail from Amazon SES over SMTP because Amazon throttles requests by emails per second and listmonk:

1. does not allow us to throttle an SMTP connection in those terms
2. will try to send messages faster than a new SES account is likely to permit (with the default listmonk config)

If sending is throttled then some messages will not be sent on the campaign, even though they could be if listmonk would wait a little longer.

Amazon responds with a transient error when throttling: 454

With the SMTP interface, Amazon SES responds with “454 Throttling failure: Maximum sending rate exceeded” after the DATA command.

source

Describe the solution you'd like

The best solution for me would be to make it real easy to run listmonk-messenger with listmonk, that way I would get complaints, etc coming back into listmonk.

A different good solution would be for listmonk to understand that a 454 error with the words "Throttling failure" means that a message should be re-queued without consuming a "retry" and maybe the sending should be paused for a second.

An okay-but-not-great solution would be to allow me to specify my sending rate in emails per second in listmonk.

Thank you for maintaining listmonk!

Related issues: #2272 #1332

[knadh]

You can throttle and set granular rates in Settings -> Performance.

[cmcaine]

Thank you, that's helpful. It's a bit confusing to me that that setting is on the performance tab rather than on the SMTP tab with what look like other connection controls.

I still think it would be nice for one of the other two solutions I mentioned to be included.