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I'm not sending these e-mails; someone or something else is doing it.
i had the same issue with my gmail. at first i thought it was the mac mail that went bezerk.
however i logged into the gmail web interface (after many years) to find that the bottom bar said last login from china, and i am based in the uk.
none the less. first thing was to change my password and enable two step verification which is brilliant.
secondly went to vacation mailer settings and deactivated them. as they were activated and sending out mails automatically to folks that were on my list.
after that created a rule in mail that sent an apology to all those that received adverts to buy viagra from my mail address.
hope this helps.
junz.
p.s. enabled a self signed certificate in keychain access as well. all those folks that i mail frequently have accepted the certificate and it can only attach itself through mail from apple, not web based interfaces. so that its all safe and secure.