Was it some kind of spam? One possibility is that someone has access to your account. The best thing to do about that is immediately change your password. Another possibility is that there is malware on your computer manipulating it. This article lists a variety of tools you can use to supplement your regular security software in checking for and removing malware: [[Troubleshoot Firefox issues caused by malware]]. Another possibility is that at some point, someone got a copy of your address book, but they are sending the messages from a different account. Unfortunately, it is easy for spammers to "forge" your name and email address as the sender of a message. Or maybe it's a coincidence and spammers are randomly mixing and matching from and to addresses to see if they can slip past the filters? I think without more information from the recipient(s), it is difficult to know which of these is the case. For example, the recipients could compare the message headers for a real message and a spam message and compare the servers the messages passed through. Spam sent by automated means from elsewhere on the internet usually travels through a different set of servers between the sender and recipient. That's what I would expect to find. If they are identical, then I would be more concerned about malware or an intruder on your account.