I am having some crazy trouble generating a Let's Encrypt cert and I think I've found the issue but I am unsure how to fix it...

I am unable to access my mail server via it's public IP but I am able to with the FQDN.  Would you happen to know why this might be?  It's a bit puzzling... you'd think if you could access via FQDN you'd be able to access via the public IP it's pointed to.

Thanks!