Email errors are NOT reported in the side conversation!

I created a support ticket for Zendesk, but they asked me to post the issue here.

LIMITATION:

Our agents are not alerted (in their side-conversation emails) when target email has errors like this one (or similar):

Error "550 5.1.1 User Unknown

In other words, it seems that Zendesk is filtering-out these errors, and this is NOT good for our workflows. As there is no reply, the ticket will remain in PENDING status forever instead of returning to OPEN status with error clearly reported!

And, here is the ANSWER from Zendesk:

Our dev team had replied to our question internally and confirmed it is expected, currently we have a limitation and those responses are not added to side conversations. Devs are internally chatting about it, but a solution is not yet in the roadmap.

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Thanks,
Marco