Email delays during bulk sending causing 2FA issues – SMTP Sender Job vs sys_email bottleneck?

[NaokiS]

Hi all,

We are experiencing email delivery delays in our ServiceNow instance and would like to better understand the relationship between SMTP Sender Job, sys_email queue processing, and SMTP server behavior.

■Current situation
- A large number of emails are generated at once due to a specific process
(tens of thousands of emails are created, and it takes about 1.5 hours to complete sending)
- During this bulk email processing, two-factor authentication (2FA) emails are delayed,
causing users to be unable to log in
- Generated emails are stored in the sys_email table and processed sequentially by SMTP Sender Job

■Suspected causes
We suspect one or a combination of the following:
1) ServiceNow-side processing (queue handling, job concurrency, etc.)
2) SMTP server limitations (rate limits, connection limits, etc.)

■Questions
1. What are the best practices to improve email throughput under high-load (bulk email) conditions?
If you have faced similar issues, we would appreciate learning what measures were effective.
(e.g., adding SMTP Sender Jobs, tuning glide.email.smtp.max_send, etc.)

2. In real-world implementations, where is the most common bottleneck?
- sys_email queue processing
- SMTP Sender Job execution
- External SMTP server limitations

3. What are recommended approaches to identify the bottleneck?

4. Are there any design strategies or workarounds to ensure that 2FA emails are delivered without delay,
even during bulk email processing?

We are trying to determine whether we should scale ServiceNow-side processing
or focus on SMTP server tuning.

Any real-world insights or experiences would be greatly appreciated.

Thanks in advance.

----- Japanese summary -----
大量メール送信（数万件・約1.5時間）により、二要素認証メールの到達が遅延し、
多くのユーザがログインできない事象が発生しております。
ボトルネック特定と対策を知りたいです。