If you were to consider the mail service for an entire nation, you’d think that it worked. But if you were to make a more detailed investigation, you’d be dealing with transportation problems, staffing issues, vehicle accidents, building fires, dogs or bears or spiders blocking access, and myriad other details which can mean mail gets delayed. The mail system would be interfacing with different and incompatible couriers in various cases, too.
It’s all more complex than it might initially look. And the web far more fragile, too.
As for your mail server, this can be the server changed names—I’ve had that happen—or has discontinued insecure access, or is just offline. Or the mail server is now requiring SSL security, and SSL-related ports. Or has been happening with OS X releases from around 10.10 os so and earlier, requiring security that those older OS X release lacks. Or the mail server switched to IMAP, though that wouldn’t usually cause problems for SMTP/ESMTP access.
Inter-operations with the rest of the internet is dragging us all forward, too. Whether we want to upgrade our systems and our apps, or not.
As for why something that worked perfectly and has stopped, that’s what you’re going to be learning more about. Compare your POP and SMTP settings with those from the provider. If you choose to identify the mail provider, and your POP/IMAP and your SMTP/ESMTP settings, we can address those more directly.
