If you go to Settings>Passwords & Accounts, tap on the Comcast account, then tap on the account again, that will show you your account settings. Under Incoming Mail Server, you should see Host imap.comcast.net, User Name is the part of your email before the 'at' sign, then your password. under Outgoing Mail Server, it should show smtp.comcast.net. to verify that setting, you need to tap on the SMTP, and just make sure it is turned on, then tap on it and makes sure you have username and password entered, Use SSL on, Authentication Password, and server port 587. Go back, tap on Advanced, and under Incoming Settings, Use SSL on, Authentication Password, and Server Port 993.
If you get confused by these settings, then just delete the account, power cycle the iPhone off and back on again, then go back into Passwords & Accounts, and add the account back new.