I'm using Mail, and will receive, but can't send e-mail. All of a sudden, about a month ago, it stopped. Have Comcast (and they were helpful, but not their issue) and am running Mojave on an iMac. Been using Mail since OS 9!!!!

I've had the same problem. But I got it solved on my end.

It turns out it IS Comcast's issue.

Login to your email account on xfinity.com, then click on the gear at the top right corner and select Email Settings. Afterwards, click on Security. On the subsequent screen, click on the Third Party Access Security checkbox. When I did that, I was able to set up my Comcast account in Apple Mail.

I've had a pair of Comcast email accounts set up in mail since 2015. Before that in Outlook and Thunderbird for several years. My experience is Comcast is horrid at supporting email in any way except logging into their website. I've always had continual problems with server issues, passwords being forgotten ... you name it.

My solution was to abandon Comcast entirely except for keeping the primary user account for them to send me notices. I get a notice from Mac mail about weekly saying my password isn't correct. I've learned ... just ignore the nags as they always go away and I never change the password yet it works again.

Here are the proper server settings from their website:

I've never changed anything and yet services come and go reguarlily. And it's always been that way.

I agree with your assessment of Comcast's email service. My primary email is provided by Earthlink but Comcast email also comes with their service package. I had it for years (all I ever received were spam messages) and it worked fine. Then suddenly about a month ago. It no longer worked. The normal solution is to delete the computer's email account and create a new one. Long story short, it has never worked in spite of all my efforts and multiple reinstallation's. For some reason Comcast prefers that browsers be used to access their server. It has become clear to me that they have a technical problem of some sort.