Testing SMTP Server Error Responses

[IBlasterus]

Sometimes during development you need to write logic for error responses from the SMTP server.
Can you add this feature to Mailpit Test Server?
There could be some startup keys that make Mailpit respond with an error.
For example, 451.

[axllent]

Previously a "chaos monkey" feature was requested (see #110, #144 & #268) to return SMTP errors at random. That proposal was rejected at the time because it was impossible to implement (properly) due to the fact that Mailpit was reliant a third party library for SMTP. Whilst Mailpit is still using a version of that library, it is now within the Mailpit codebase meaning it is now technically possible.

What you are asking for however is something different, so to help me understand your requirements:

1. Are you wanting a 100% chance of failure in your tests, or at random?
2. Is there a specific type of failure in the SMTP transaction, or is any SMTP error OK?
3. Should the error result in a SMTP server disconnection, or...?
4. Anything else you can think of...

The example given previously was MailHog's Jim feature which is what many/all of these previous requests were based on, and which may help provide some suggestions and ideas for this discussion.

Just to be clear, I'm not saying (yet) whether I will add this option, I'm just a lot more open to it now because it is now possible, it just needs to be implemented properly which takes a solid understanding of the requirements, and of course planning. It is also worth mentioning that I am having shoulder surgery in 2 days, so if I am to add this option it would likely only be in January or February next year, which gives me plenty of time to consider the options before then.

[IBlasterus]

Thank you very much for taking the time to answer my question.
I wish you a speedy recovery.

1. Are you wanting a 100% chance of failure in your tests, or at random?

Especially for me need 100% chance of failure.

2. Is there a specific type of failure in the SMTP transaction, or is any SMTP error OK?

Need specific type of failure. For example, for development backend reaction on 451 error.

3. Should the error result in a SMTP server disconnection, or...?

Maybe. My Backend connect to SMTP, send E-mail and disconnect.

4. Anything else you can think of...

Nothing else comes to mind.

[ThomasLandauer]

Just an idea on how to implement this:
Hard-code an email address like [email protected], and if it's used as sender/receiver, return that error.
Any more fancy way (like sending some special keywords in the SMTP session) won't work, since most users probably can't modify that.

[axllent]

Now that's an interesting approach, thanks for the idea @ThomasLandauer 👍

Edit: After some thought and as nice as this idea sounds, it is very limited in that the email would need to have already been processed in order to process the addresses, meaning the only failure one could trigger this way would be after a successful transaction.