It sounds like more than likely you could be behind a firewall that is not allowing your computer to communicate to our server over port 993 possibly. However this port is used for a secure connection to the IMAP server which is for receiving mail.
You can typically just login to webmail to send out any mail as this works over the standard port 80 along with normal website traffic if you're having any issues connecting in your mail client because of a firewall. However the port you should be using for secure outbound mail would be port 587, so that might be all that your problem is.
If you have any personal firewall or anti-virus software running on your computer, this could be blocking a connection, so you might want to temporarily disable those as you test.
Please let us know if you're still having issues.
Thank you for that info. The solution to my problem turned out to be in MacMail: with every message I want to send, I must confirm and possibly reset the "from" email address and OGM server.
Mac Mail was suddenly sending to the wrong server for a given "from" email address. My job requires me to maintain (and send from) three email addresses. Also, I took your advice and changed the OGM setting to port 587 only (it had included two other possible ports).