The options from Mavericks Mail and Yosemite Mail are different. However, I see you have Gmail as POP. Look under Mailbox Behaviors tab to see if there is an option there to download complete messages. I no longer have any POP accounts to check settings.
I highly encourage you to setup both Gmail and Comcast accounts as IMAP. In Yosemite Mail, by default Apple sets you up with Gmail and Comcast as IMAP.
Before removing your POP accounts...
1) Select to move all messages in the Inbox(s) to a local folder or select Archive. When you delete a POP account it also deletes all messages in the Inbox.
2) Clean up the SMTP settings.
EDIT SMTP SETTINGS
In Accounts window > Outgoing Mail Server SMTP: select to Edit SMTP Server list... from popup.
Remove all SMTP servers listed. When you add accounts they will add the new settings.
Use Gmail with Mavericks Mail **
http://www.needhelp4mac.com/2013/12/use-gmail-with-mavericks-mail/
Remove the POP accounts and add back one at a time as IMAP so you can troubleshoot if needed before adding other accounts.
In case Mavericks Mail does not auto setup Comcast as IMAP, you can use these settings.
Comcast IMAP Settings
Incoming server: imap.comcast.net
Outgoing server: smtp.comcast.net
Authentication ON
Incoming Port: 993 with SSL ON
Outgoing Port:
465 with SSL ON
**(Fair disclosure: NeedHelp4Mac is my site. I may receive some form of compensation, financial or otherwise, from my recommendation or link.)
