Use SMTP pool to send emails

[ben-z]

Today, we were sending 558 emails at the same time after introducing the "active terms" feature. SMTP started throwing this error:

│ Error: 454 4.7.0 Too many login attempts, please try again later. For more information,
│ 4.7.0 go to
│ 4.7.0  https://support.google.com/mail/answer/7126229 00721157ae682-6f3e74459a1sm64557097b3.46 - gsmtp
│ 
│   with email_email.all_emails["o-zhang-05-expiring-soon"],
│   on common_emails.tf line 9, in resource "email_email" "all_emails":
│    9: resource "email_email" "all_emails" {

Only 188 out of 558 emails were sent in the first round. In the second round, slightly over 100 emails were sent.

It turns out we are creating a new SMTP connection for every email, which is not desirable:

https://stackoverflow.com/a/54299705/4527337

This ticket tracks using an SMTP pool to send emails.

[ben-z]

https://github.com/knadh/smtppool may be a useful project.

[ben-z]

There appears to be a new object-oriented way to define resources: https://developer.hashicorp.com/terraform/tutorials/providers-plugin-framework/providers-plugin-framework-resource-create

This makes it easy to persist an SMTP client for use multiple times.

[ben-z]

The smtp.SendMail function is just a wrapper for working with an underlying client. We can use the client directly and persist it for multiple emails.

https://github.com/golang/go/blob/608acff8479640b00c85371d91280b64f5ec9594/src/net/smtp/smtp.go#L321-L376

We might need to manage concurrency so that smtp messages don't send interleaved.