You just got an email from someone in your address book saying that they got an email from you that you did not send about being in touch- the message does not appear in your outbox. help?
This is a typical case of e-mail address spoofing. There is
nothing you can do about it, some spammer forged the header and
made it look as if you sent the e-mail. The spammer don't even need
access to your e-mail account, spoofs like these are mostly done
outside the e-mail system of the spoofed victim.
If your e-mail address is spoofed a lot, you will receive a lot
of bounce backs to e-mails that you did not send.