This is caused by your email settings, the system is trying to send an email from one address on behalf of another.
Check your user profile settings and the Email options to see which email address it is trying to use:
Click System
Click User Records
Click on the affected user that is trying to email
Click E-mail
Check the Email field and check that the SMTP settings match to this email account, or that the email address in the SMTP settings has access to send emails on behalf of the user.