No need to call an exorcist!
If you get an email from the MAILER-DAEMON; this is just an automated message sent from an Email Server somewhere in the World. Usually you only hear from Email Server's when they have trouble delivering an email you sent or an email that shows your email address as being the sender.
A 'Daemon' is a software program that works behind the scenes on a Server, doing useful stuff.
When you get a MAILER-DAEMON@whatevercompany.com message in your inbox, the Mail Server at that company is informing you that it is returning your message because of some failure to deliver it. For example, the "To" email address may no longer be valid, or there may be a problem routing the message to the appropriate Mail Server.
Chances are you never even sent the message in the first place! Your email address could have easily been copied and used as the email's "From" address without your knowledge. This is a well known technique employed by Spammers.
Rather than using their own email address in the "From" field, they use your email address instead. In this case, you would receive any failure notifications from various Mail Servers if they have been unable to deliver the email to the recipient.... it is these failure messages that are named MAILER-DAEMON!