I am using Mac mail and when I hit send it chugs along with a blue line in the lower left and eventually does get sent. Even a 6kb email is delayed. A few weeks ago I sent an email to 9 people 6 in the "To" line and 2 in the "cc" line. I got a message immediately that it did not send. However, within 2 minutes I got an auto reply that was a vacation response from the first person in the "To" list. I checkdd with the Cc'd individuals and only 1 of the 3 got it and it was the last one. I immediately went to the sent folder and it was not there. I went to the Xfinity account and it was not there either. I double checked the address of each and they were correct. Several hours later I reconstructed it and sent it and the original was still not in my sent folder. 3 days passed and the original was in both mac mail and Xfinity but I have no idea when it appeared.
It is NOT my apple product, but rather something going on in the Comcast server. It is happening to too many different user with different mail programs to be all of us.
Disable the account in Mac Mail, then sign into the account using only the webmail and your browser. Now send a test email to the same recipients. Does it work OK?