[GH-ISSUE #592] Delivery delay when using Mailpit as SMTP target for Amazon SES #379

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opened 2026-03-15 14:09:14 +03:00 by kerem · 3 comments

kerem commented

2026-03-15 14:09:14 +03:00

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Originally created by @Nchona on GitHub (Dec 10, 2025).
Original GitHub issue: https://github.com/axllent/mailpit/issues/592

Mailpit version: v1.27.11 (Docker, behind AWS NLB on port 25)
We are observing consistent delivery delays only when emails pass through Amazon SES before reaching Mailpit.
We’re using Mailpit as an SMTP sink for tests that send emails via Amazon SES.
When we send directly to Mailpit, messages appear in the UI almost instantly.
When we send the same test via SES → Mailpit, messages are often delayed by 1–2 minutes (or more) before they appear in Mailpit, even though SES returns 250 Ok quickly.

Mailpit logs only show the messages at the time they finally arrive; there are no earlier SMTP or DB logs for those messages.

Is there any known issue in Mailpit v1.27 that could cause delayed message handling/acceptance in this scenario, or any recommended tuning for high-volume SES traffic?

Originally created by @Nchona on GitHub (Dec 10, 2025). Original GitHub issue: https://github.com/axllent/mailpit/issues/592 Mailpit version: v1.27.11 (Docker, behind AWS NLB on port 25) We are observing consistent delivery delays only when emails pass through Amazon SES before reaching Mailpit. We’re using Mailpit as an SMTP sink for tests that send emails via Amazon SES. When we send directly to Mailpit, messages appear in the UI almost instantly. When we send the same test via SES → Mailpit, messages are often delayed by 1–2 minutes (or more) before they appear in Mailpit, even though SES returns 250 Ok quickly. Mailpit logs only show the messages at the time they finally arrive; there are no earlier SMTP or DB logs for those messages. Is there any known issue in Mailpit v1.27 that could cause delayed message handling/acceptance in this scenario, or any recommended tuning for high-volume SES traffic?

kerem

2026-03-15 14:09:14 +03:00

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kerem commented

2026-03-15 14:09:30 +03:00

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@axllent commented on GitHub (Dec 10, 2025):

Hi @Nchona . There is no known issue with Mailpit as it does not throttle (it's actually designed to be as fast as possible, receiving/processing 100-200 messages per second).

I strongly suspect the delay is caused by something like SES queuing messages internally and distributing it via worker nodes. Do the email headers not provide clues as to where the delays are coming from?

@axllent commented on GitHub (Dec 10, 2025): Hi @Nchona . There is no known issue with Mailpit as it does not throttle (it's actually designed to be as fast as possible, receiving/processing 100-200 messages per second). I strongly suspect the delay is caused by something like SES queuing messages internally and distributing it via worker nodes. Do the email headers not provide clues as to where the delays are coming from?

kerem commented

2026-03-15 14:09:35 +03:00

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@github-actions[bot] commented on GitHub (Dec 18, 2025):

This issue has been marked as stale because it has been open for 7 days with no activity.

@github-actions[bot] commented on GitHub (Dec 18, 2025): This issue has been marked as stale because it has been open for 7 days with no activity.

kerem commented

2026-03-15 14:09:40 +03:00

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@github-actions[bot] commented on GitHub (Dec 21, 2025):

This issue was closed because there has been no activity since being marked as stale.

@github-actions[bot] commented on GitHub (Dec 21, 2025): This issue was closed because there has been no activity since being marked as stale.