In the meantime, my husband found that our primary account had received a note that our computer must be infected with a virus because it was creating spam. Comcast had blocked port 25 for outgoing email to prevent this activity. Naughty me had sent out messages a few nights earlier as part of a needlework ornament exchange. Thirty-one messages in 25 minutes at a somewhat regular rate (I had a routine copying and pasting between three files to create the messages) had triggered their virus detector.
But Macs don't have any wild viruses at this time. My wireless network is locked down. My machine was not spamming.
But to be able to send out mail, I needed to use a different port than the default port 25 I used when I set up my Comcast account years ago.
I needed to change the settings I was using for outgoing mail so that Mail uses port 465 with SSL and authentication for outgoing mail. An alternative is to use port 587 without SSL. The top image below shows the Preferences / Accounts info for OS X 10.5.
If you go to the Comcast help forums and look in the Comcast High-Speed Internet and then Macintosh forum, Joel has pictures of how to set-up and adjust your mail settings in 10.5. Look in the top 'sticky' threads above the solid line or try this. That forum is a useful place to get help.
For OS X 10.5, open Mail and Preferences. Click the Accounts tab. The examples here are for existing accounts where you already had Mail working with Comcast. Click on that account (called comcastAcct here) and you'll see a pane like this. For the example, I am using acct1 as the Comcast user name.
Click on the Outgoing Mail Server name and pull down to Edit Server List. That brings up the next panel where you need to choose Advanced to get the pane shown.
The middle of that pane may now look like this:
You need to enter 465 as the server port (or just click the defaults button), click on Use SSL, make the authentication Password and type in your Comcast username (acct1 here) and password. Click OK.
While you are at it, check that your incoming mail is also a secure connection. Go back to original Account Information pane (top picture) and click on the Advanced tab. Set your incoming port to 995 and click Use SSL with Password authentication. That is shown below. Note that you do not need to change your other settings at the top about when to remove mail from the server. That is your choice.
This is an update all Mac users probably should do now rather than wait for Comcast to decide you're spamming. Since making that change, I've had no problems sending mail.