It appears that many messages I send to customers all over the world are being rejected as undeliverable . . . as spam . . . (which is not the case, they're customer invoices).

Here's a sample of what the return message says :

SMTP error from remote mailer after MAIL FROM:<****@thenorthfaceguy.com>: host ncmx03.mgw.rr.com [24.25.4.97]: 553 5.3.0 Mail from 67.15.16.19 rejected,
see http://spamhaus.org/sbl

Here's another sample :

host mx2.mail.spray.net [212.78.202.7]: 554 Service unavailable; Client host
[67.15.16.19] blocked using sbl.spamhaus.org;
http://www.spamhaus.org/SBL/sbl.lasso?query=SBL15926

I don't know jack about spam, and I am anything but a spammer -- basically some other clown on the same server is screwing things up for people.

Please advise -- now what?

Due to outgoing spam server 20 was blocked in spamhaus.org.
We've resolved this issue and our ip address is unblocked:
http://www.spamhaus.org/SBL/sbl.lasso?query=SBL15926

Sorry for the caused inconveniences.