I've got it solved, with your help.  I believe the problem was in the password. Apparently I have trouble typing and spelling.

I had the settings pretty much as you suggested.  I have two comments on differences.  Port 587 will not work on my system.  It gives me an error message concerning SSL.  I'm using port 465.  In the bottom section, 2nd option, I enter only my user name, not the full email address.

I thank you very much for your help and guidance.

I've been out of town for two months, and returned last Friday.  Something happened to my Pegasus while I was away. 

I can receive mail with no problem.  I'm unable to send any email through Pegasus.  I can, however, send email through my ISP, Comcast.  When I try to send through Pegasus I receive an error message, as follows:

 17:29:41.628: --- 28 Oct 2018, 17:29:41.628 ---
17:29:41.628: Connect to 'smtp.comcast.net', timeout 180 seconds.
17:29:45.107: [*] SSL/TLS session established
17:29:45.107:

... 
17:29:45.341: >> 535 5.7.0 ...authentication rejected (6)

I'd certainly appreciate some help on this.  I don't know which way to turn.

EDITED 10/29/2018 to remove creds

Your credentials are included in your post in easily decodeable format.  You should make changing your credentials your first priority.

Unfortunately I can't help you with Comcast specifically; only to suggest you confirm your smtp host configuration with current Comcast specs.

I just spent a hair under an hour with the Comcast "advanced tech" people trying to resolve this.  I spoke with a very nice young lady who did her best.  She assured me that the SMTP configuration is correct, though she did admit that they do not have their normal "cheat sheet" for Pegasus.  They simply do not list Pegasus.  Her strongest suggestion was that I verify the port settings.

I'm using port 110 for POP 3 incoming mail, and port 465 for SMTP outgoing.  Are these correct, or might there be some better?

 I'm using Windows 7 and Firefox 61.0.2.  Firefox is slightly out of date because I've been out of town for 2+ months.

 What all is involved in changing my credentials?   What must I do in that regard?

Based on what I can find via a web search, try these settings for you SMTP host:

On the General tab:

Server host name:  smtp.comcast.net

Port: 587

Timeout: 180

Leave the remaining options unticked.

On the Security tab:

In the top section select "via direct SSL connect".  Leave all other options blank.

In the bottom section select the second option and enter your full email address (username@comcast.net)  and the password