mysterious emails not sent by me

Several contacts have emailed me to say they have received a post from me with only a line of letters and symbols. These contacts are random, some I've posted to years ago, others only once, but friends as well.
The line of letters and symbols seems to vary. Here are three of them:

I've scanned with Microsoft Security Essentials, Asquared and Spybot, with totally negative results. My emailer is Gmail.

  Woolwell 11:40 17 Mar 11

Probably not originating from you at all but from someone who has managed to find your e-mail address and is now spoofing it in an attempt at spam. Regret that there is not a lot you can do about it. It may die down. To be on the safe side change your gmail password. Long term if it continues you may have to change e-mail address.

Thanks, Woolwell. I'll try that

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