I wrote a previous post regarding saving messages sent via POP3 accounts into Gmail’s sent items yesterday and it can be read here. To expand on that there is another option to accomplish the same goal which is to set your outgoing mail server as smtp.gmail.com and use your Gmail account settings to authenticate. This second option is probably better since it will standardize how you are sending mail at all times. and more than likely sending through Gmail is more secure than with a typical SMTP server depending on the SSL options offered with the SMTP server account.
One thing to watch out for if using this second option is duplicate messages in your IMAP Sent Items folder. This problem can be easily resolved by following the below steps.