mail sends incomplete msgs before I click Send! (Mail4.6/1085)

I have an iMac and a MacBook, same os level, same mail level. Email is a gmail account. I use it on both my computers.

My imac keeps getting partial emails whenever I start an email from my laptop. Are there any mail settings on the laptop which can stop that behavior?

And what is the "Use IDLE command if the server supports it."?? (This box is not present on the imac version of my Mail preferences/Advanced tab, only on the laptop Mail app.)

Thanks

S