I have same problem. I can send and receive just fine from home, but at work I can not log on to the COX SMTP server. Same version of Mac Mail with all the same settings on both home and work machines.
The reason that shows up in my Mail Log is:
READ May 01 12:30:16.903 [kCFStreamSocketSecurityLevelNone] -- host:smtp.cox.net -- port:587 -- socket:0x6000006a11a0 -- thread:0x608001264900
554 cxr-obgw-5003a.stratus.cloudmark.com cmsmtp 140.198.32.9 blocked. Refer to Error Codes section at www.cox.com/.../email-error-codes.html for more information. AUP#CXDNS
I've gone to many sites (including spamhaus), and my IP address is not listed as a BLOCKED IP address anywhere?
This started happening April 24...coincidentally(?) with COX's change to mail security protocols.