Not receiving emails from some senders

Thank you for your response XfinityLinda.

My setup under Security has all three boxes checked, including the "Third Party Access Security" checkbox, which is my preference. I'm not sure how that would affect my ability to receive all emails. This is not unique to my Apple Mail access but more importantly to my Xfinity Email website access. How would unchecking that box influence what is being delivered to my Xfinity Email?

@g.maj Did you review the FAQs from my link? I'll share here for the benefit of the community. 

FAQs

Can I still access my Xfinity Email account (Comcast.net) through a third-party program after I check the security box?
Yes, you can manage your Xfinity Email through third-party programs when the Access Security box is checked. If this box is unchecked, you’ll receive an error message when you try to set up your Xfinity Email through a third-party program.

We recommend accessing your Xfinity Email from the Xfinity Email website.

Will this impact me if I have third-party email on my Xfinity account?
No. If you already have a third-party email on your account for verification purposes, you won't be impacted.

What will happen if I don’t disable third-party access?
If you continue to access your Xfinity Email account through a third-party client, you may be more susceptible to potential credential fraud.

Will changing the email security settings affect how I receive emails or manage my mailbox?
If you don’t have the Access Security box checked, you may receive an error message when you try to set up your Xfinity Email through a third-party program. This is the only change you may experience.

Thanks XfinityLinda.

I did review the FAQs from your link. My problem however has nothing to do with third-party access. If I were to disable Apple Mail, the missing messages are still not showing up on the Xfinity Email website.

Thank you very much for the clarification @g.maj. If there is an issue with a domain being blocked for some reason you may have to reach out to the security team at the https://spa.xfinity.com/ site to have them check on that for you.