After completing the basic installation, all AntiVir jobs will add a string in the subject (e.g. Scanned with Avira Scan Engine).
You can adjust this message within a job at any time, just by modifying the text [Avira checked]. If you do not want this subject extension, you can remove it from the Job Configuration. You can find the option under:
Policy Configuration → Mail Transport Jobs → (right-click the job) Properties → Text in subject