More of a general inquiry - We've had a client recently have a number of Umbraco forms email workflows change due to a security policy change on their SMTP and the error wasn't caught for a couple of days. Part of the triage is figuring out what can be done to spot / prevent this in future and one of the thoughts was by having some sort of Fallback SMTP configuration in place. Was just wondering if this was possible in the default installation of Umbraco 13 + Umbraco Forms package, or would something custom need to be whipped up?

There's nothing out of the box for this. We catch this in custom workflows with "special logging" which is picked up by alerts. Then we resend the mail manually using the Forms backoffice, "retry workflow task" or similar. I don't think we use the ootb mail-send workflow, so we always get our alert.

Forms uses Core to send emails, so it should be possible with a custom implementation of the [IEmailSender](https://github.com/umbraco/Umbraco-CMS/blob/dfe41d7f7630b5d1be144a4ea1fd8e861143f839/src/Umbraco.Core/Mail/IEmailSender.cs) interface

Thank you both. We shall take a look at creation out own IEmailSender interface!