Cllick send; mail never makes to mail server

For some, but not all emails, I click send and it appears it is being sent but later I discover that it never went out, there is no record of it in the sent folder and the email never hits my company's IMAP server so there is no record there either. And the draft has disappeared from the drafts folder. I have used spotlight to see if the emails have been saved to some weird place on my harddrive but am not finding them. I feel like I am going mad ('I could swear I sent that email but where is it???)