Sure thing: Mail not downloading
It doesn't appear to be consistent but for those experiencing it it's a major issue.
Reminds me of my favourite macOS issue with mail.app for IMAP servers, that I've seen with two different hosts. Because modern mail.app won't let you configure all the server stuff straight away, because it's being "easy to use" and "intuitive" it insists on attempting to connect to the server and figure that stuff out with just the address and password before it'll let you configure that.
Unfortunately, it's massively over-aggressive in the way it does that, and so triggers an instant IP address blacklisting with the host, so then you go to the maual server detils entry, because it can't auto-figure the connection (due to the ban). Even if you put all the correct info it, it won't work, because your ISP-provided modem IP address is blacklisted.
So the only reliable way to configure mail with a new host is to do your initial entry on a VPN connection, let it blacklist, then switch the VPN to a different exit node / IP address, and then complete the connection setup manually.
Bozo developers, making clownshoes software.