Virus error message on MacBook Pro

It can also depend on the email server. Such as, if you added a .zip or .jpg attachment, some systems will automatically regard those as possible malware (without even checking to see if it is), and then either strip the attachment before sending the rest through, or only notifying the recipient (or both parties) of possible malware and not sending it through at all.

My oldest sister's email provider is like that. If I want to send her just about anything that would normally be an attachment, I have to put it on my web site's server space, then provide a direct link to it in my email to her. And even then, she'll get a prompt from her email service that the link she's clicking on to download the file may be dangerous.

Assuming you did not include that phrase in the subject line it means either your email service provider or the recipient's email service provider is tampering with your email communication.

I would add one more potential source to the list John Galt wrote above:

...or the recipients computer (if not a recent Mac).