Just to clarify, Dave - when Jim says to try setting up a new account, he means a new user account on your Mac, not a new email account. Sometimes things get corrupted in user accounts - this is a way to figure out if that is the issue. Just start up Mail, add the Comcast account and see if the issues are the same as they are when you're logged into your normal main account.
As for alternatives to Comcast - I have no idea, but this might help:
https://www.highspeedinternet.com/inAs for alternatives just for your email, why not use iCloud? It's free - and you're not the product, the way you are with Gmail.
Most of the free email alternatives rely on advertising to you or collecting your data, so depending on how you feel about that, perhaps not the best option. But that said, Gmail is very good - I've got quite a few Gmail accounts for various things. And it's been reliable and has good spam filtering - which is more than I can say for Yahoo and its peculiarities (my ISP, Rogers, uses it, so I'm rather more acquainted with its foibles than I'd like).
The other option is to buy a domain and cheap hosting. Then you can set up as many email accounts as you want (depending on the hosting). And if you change ISPs at home, it doesn't matter - you keep your domain and email addy(s).
Or, if you don't want to bother with that and just want the email, then check out something like this:
https://www.namecheap.com/hosting/email.aspx?sscid=b1k2_7yzkr&utm_source=SAS&utm_medium=Affiliate&utm_campaign=1383199&affnetwork=sas - you can buy the domain from Namecheap. (I recommend them as a domain registrar and have a couple of client's sites there as well - good, reliable host). Or, my other fave (buy the domain at Namecheap) for hosting - Crocweb.com* - use the HOSTME coupon and sign up for 3 years and you get 3 years of hosting for just over $60. I have moved many of my clients there now; very reliable, good tech support, based in Canada, though bills in USD. Do NOT fall into the trap of signing up for something like iPage which has an intro price of $1.99, and then renews at $8-$10 a month.
*This site is now hosted at Crocweb. We moved from Asmallorange.com - they were bought out by host-gobbler EIG and the prices went through the roof. Avoid all EIG-owned hosts (Hostgator, Bluehost and on and on - look at the list at Wikipedia before signing up with anyone, should you go that route. They RUIN hosts; they're the Walmart of hosting companies)