A spammer is probably spoofing your e-mail address (it looks as if it from you but isn't) and sending spam to random e-mail addresses. Many of these do not exist and are bouncing back to you (mailer-daemon). This should die down in due course. Precautions to take are a full virus and malware check to make sure that your system is clean, and change your e-mail password. If it persists for some time then you may have to change your e-mail address.
Sony Xperia Z5 review: Hands-on with the phone which the Z3+ should have been