I have two accounts, a gmail one and a school email address. I want to be able to send emails from my school address and gmail, but receive only from the gmail one (the gmail and school email accounts are synced together so I don't want to get duplicates). is there any way to make only the receive section of an account offline in Mail.app? I'm using OS X 10.9 Mavericks, if that helps. I already turned off automatic receiving for my school email, but if I hit "get mail" all the email from that account gets fetched anyway.
Set up a new empty email box (perhaps with server side quota or rules to delete all mail immediately upon receipt).
Enter those credentials as a new account in Mail.app and end the sending credentials and server for this shell account.
Now you can send from the account you want and have no emails coming in.
After tests, I think the least buggy method for Apple's bad implementation is setup gmail with pop3 method in Mac Mail.app and set rules to delete all incoming emails.
There are no rules for iPhone but we can turn off auto fetching.