Have you seen this sort of problem reported by a blogger?
"Someone is posting unauthorised articles on my blog"
"Someone is putting advertisments for a Porn Site on my blog"
"There are photos on my blog which I did NOT put there! Help!"
If you have, then there's a pretty good chance that the blogger's < SECRET > email-to-Blogger - Email Posting Address, or Mail2Blogger address, has been compromised. In other words, someone has either been given the address, or has discovered it, by happenstance.
Once someone has this address, they can post to your blog - at will - if your setting is Publish Emails Immediately. You can, of course, change it.
Don't give it out to people whom you do NOT trust - explicitly.
Once you have used this feature for your purposes, disable it! You can enable it again, or you could even change it, occasionally, too. This address, like your account ID, should be regarded as SECRET!