A: Hi,

Thank you for your question.
It's Alex, the founder of ProxiedMail.
If you're using custom domains I would suggest just set up ProxiedMail as only DNS and make it forward to the webmail.

ProxiedMail works well only when its the single MX & SPF records on the domain.
If you use just forwarding instead of setting up the priorities to MX servers it would work the same.

Using multiple MX servers for ProxiedMail is not well tested and supported yet.

Thank you.
Alex, the founder.