How are you accessing your email? With only the webmail interface and a browser? Or with a desktop client like Thunderbird or Outlook? Or with the mail app on a mobile device? Exactly what happens when you try to send? Are you getting any error message?
I have the same outbound problem with comcast. Using Thunderbird as email handler. Contacted comcast to verify smtp settings. All were good.
Thunderbird worked great 2 days ago, now it won't let me send anything. I tried to use same comcast account on my ipad email program, and it got blocked as well (worked fine 2 days ago as well). So I'm thinking it's a comcast problem, not a software problem on my end.
I've deleted the entire comcast acct and re-established it, but no joy.
I do not get an error message I just have all my emails sit in the outbox.
It works on my cell phone not connected to the comcast modem.
Thanks for helping...
OK, I'm using Mac Mail on an iMac, and I cannot duplicate this problem. Does it work OK if you use only the webmail interface and a browser? What version of Mail are you using? I have v12.2 (Mojave).