My company deals with thousands of transactional (not marketing) emails each day.

Comcast is one of the worst for deliverability. Many emails sent to Comcast accounts never generate a bounce but also never actually show up anywhere in the recipient's email account (inbox, spam folder, etc.). They just disappear into the ether.

My experience both at work and at home is that if you want a reliable (and free) email address that both minimizes email deliverability problems and also pretty much effectively blocks spam (without many false positives), you should use Gmail.

In general, there's rarely a good reason to use an ISP-issued email address. Email is not a profit center for ISPs, so they don't invest in good infrastructure. The back-end processes are poor and the front-end UI is usually poor as well. The only reason the ISPs even still bother to provide email accounts is because they know it increases friction for users who want to switch to another ISP. If another company comes along with a better speed, a better price, better service, etc. and you decide to cancel Comcast, it's going to be a massive headache to log into every site you've ever done business with and update your email address to your new account. The hassle involved with that means that you may think twice about switching, even if the other ISP is better.

Email service providers that focus 100% on email tend to do a better job developing and maintaining their infrastructure as well as being on the forefront of technologies that serve to maximize the delivery of legitimate email and minimize issues with illegitimate messages (spam, etc.), like SPF/DKIM/DMARC/etc.

I'm currently trialing Fastmail. It's a paid service, but they seem to offer an very reliable, rock-solid, and customizable product, plus (since it's their main business and revenue stream) they are on the cutting edge of support for new developments in email technology and standards. It lacks a few of Gmail's bells and whistles (and I'm missing Gmail's "All Mail" mailbox and search functionality), but I've appreciated a lot of the settings I can tweak under the hood to make it work how I need it to. I doubt I'll make it my primary email (I'm too reliant on Gmail's search function to keep my life organized), but it has its uses for me as a secondary account and works better than just about any other non-Gmail provider I've seen (and better than Gmail for a few things).