I would like to suggest a feature that allows users to configure their own Custom SMTP settings within the "Custom Domain" management area.

Currently, replies are sent via SimpleLogin's infrastructure. However, I believe it is best to send email replies using our own SMTP servers. Ideally, SimpleLogin would connect to the user-defined SMTP credentials to send the reply.

I suspect this feature hasn't been implemented yet due to concerns about SMTP abuse. However, enabling users to "Bring Your Own SMTP" would actually shift the reputation risk away from SimpleLogin's IPs to the user's own providers, effectively mitigating abuse on your platform while improving deliverability for power users.