Spammers using my account?

Spammers using my account?

Hi,I've noticed that I've started to receive lots of "Mail delivery failed: returning message to sender" messages. All of them coming from email addresses I dont even recognize. One typical gmail.com message starts with:
" niXXXXXXXXXXXXXXXXX@gmail.com 
host gmail-smtp-in.l.google.com [64.233.180.26] 
SMTP error from remote mail server after end of data: 
550-5.7.1 [199.217.112.118 12] Our system has detected that this message is 
550-5.7.1 likely unsolicited mail. To reduce the amount of spam sent to Gmail, 
550-5.7.1 this message has been blocked. Please visit "

What should I do?

Jozsef