If in email properties and source the sender is not you then your email is being spoofed. A spammer or trojan infected PC is sending emails saying they are you. You then get
1 emails from irate people thinking you are spamming them and
2. Emails from postmasters saying email undeliverable
I got 200 or so undeliverable ones about 12mths ago all to Russian email addresses seems someone who had my email in their address book got infected.
It all died down in a few days. Nothing you can do except hope your email not tagged as a spammer and blocked
Your email account has been hacked, it happened to me. Could have happened in a number of ways. I know how it happened to me its like a chain reaction I received an email that appeared to be from a trusted source I replied to it asking if it was really from them. The reason I suspected something was wrong with it was the amount of recipients including the person entire address book which was unlikely. Then this virus read my address book and did the same thing and so on and on it goes.
I ended up calling Microsoft for help & downloaded their free PC scan and then created a case. They spent hours with me trying to get rid of the last 2 stubborn infections which didn't seem to want to leave. Finally the computer scanned clean and my hats off to Microsoft for all their help and it was free.
I hope you didn't just leave it alone Microsoft advised me to immediately change my password and send a warning message out to my contact not to open anything from me unless the subject line said "VALID" or something similar, hope you do the same.