I responded the the Craigslist ad, asking about the small block covers, & the email bounced back to me.
----- The following addresses had permanent fatal errors -----
<
[email protected]>
(reason: 554 5.7.1 <unknown[216.255.50.19]>: Client host rejected: rDNS/DNS_validation_failed._Please_setup_matching_DNS_and_rDNS_records:_http://www.craigslist.org/about/help/rdns_failure xabd-4)
----- Transcript of session follows -----
... while talking to mxi4p.craigslist.org.:
>>> DATA
<<< 554 5.7.1 <unknown[216.255.50.19]>: Client host rejected: rDNS/DNS_validation_failed._Please_setup_matching_DNS_and_rDNS_records:_http://www.craigslist.org/about/help/rdns_failure xabd-4
554 5.0.0 Service unavailable
<<< 554 5.5.1 Error: no valid recipients