Huge number of swbell.net address probes - anybody else?

My host is being hit with a huge number of email address probes from swbell.net addresses. They are all like:

May 4 09:55:50 habanero postfix/smtpd[9685]: reject: RCPT from adsl-68-94-181-200.dsl.rcsntx.swbell.net[68.94.181.200]: 550 <nanette@[mydomain].com>: User unknown; from=<joe@nowhere.com> to=<nanette@[mydomain].com>

but the name in the "to" address changes each time. I guess they are just looking for valid email addresses. I've gotten about 4000 of these since Sunday. Anybody else? Short term I've blocked swbell.net for port 25 but I can't keep that around for long. I contacted their abuse department but have not heard back yet.

 

 

 

 

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