Email's not being sent from HelpDesk

For all three sites across 2 different Microsoft 365 tenants, emails come in to the helpdesk, ticket gets created, but no email is sent out.

This happened before. It took 16 hours to respond. If it takes that long again, we will be looking for a new Help Desk.

Hello! We have fixed the issue for now and are investigating what caused it. All unsent notifications either are already sent or will be sent shortly. Please confirm that for now, all works as expected on your side. I am sorry for the troubles you encounter and will inform you about the results of investigating the issue.

This is still not working.

Thank you for the feedback, I have informed developers about the continuing issue and they are working on it with the highest priority.

The developer detected in logs errors related to Outlook 365. We advise you either to delete the connected Outlook 365 mailboxes and re-connect them again or use the default SMTP settings according to the description in the documentation. Please try the suggested options and let me know whether any of them helps.

We deleted the custom SMTP settings and saved.

Then entered them again and did a test. We get the message 'Email settings are valid'.

Then we refresh the screen and we get the error 'We noticed that something is wrong with sending emails. Please check SMTP settings. The last detected error: Couldn't authenticate. 451: 4.7.0 Temporary server error. Please try again later. PRX4 [].

I am sorry for being unclear. I meant Outlook 365 mailboxes that were configured according to this instruction. Open this URL:
https://{your helpdesk}/HD/pages/outlook365.aspx

Then, delete all the mailboxes you have there. Go to Settings > Email and configure the custom SMTP again. Check whether the issue persists. If it does, please delete the custom SMTP settings and check again. Let me know about the results you have.

That didn't solve the problem, made things worse.

We are done wasting time on this. It is better spent on replacing this product with something that has competent support.

Thank you for letting me know that support is not required anymore. I just would like to draw your attention that the product is highly integrated into Microsoft 365 environment and depends on workability and preferences of your tenant services. Problems that appear in HelpDesk are not necessarily bound with HelpDesk only and it mostly requires united efforts of a client and the support to find out the root of an issue and solve it.