I can receive email with Comcast but cannot send it

That sound like a problem with the SMTP settings. Go to Settings > Mail > Accounts, and tap on the account giving you trouble. Scroll down on that page and check the setting under SMTP. It should show "smtp.comcast.net". If that one is showing there, you can tap on it and check the settings. Start by entering your password on that page, this is the password you would use to login their website at www.xfinity.com. Also the Use SSL setting should be checked, the Authentication should show "Password" and the Server Port is 587.

Was this account recently set up or did you just start having the issue?

Is the account using POP3 or IMAP?

The settings are usually different depending on if you are using POP3 or IMAP. POP3 is when you can delete an email on one device and it is not affected on another device using the same email. When using IMAP, deleting on one device will also remove it on any other device that uses the email. It does show on the settings screen after you tap the Account whether it is being set up as POP3 or IMAP.

It may help if you can provide screenshots of that settings page as well as the SMTP settings page, just make sure you block out your email address.