On trying to send mail to one of my co-Developers, I go this answer:

  SMTP error from remote mailer after MAIL FROM:<$myid@not.so.argh.org> SIZE=2849:
  host relay.verizon.net []: 550 Email from your Email Service Provider is currently blocked by Verizon Online's anti-spam system.  The email "sender" or Email Service Provider may visit http://www.verizon.net/whitelist and request removal of the block.

Basically, this means: unless you are a large provider, you cannot send mail to verizon's customers. Of course, mail is something in both directions: Who wants to receive mail from me should better not use a Verizon-address, otherwise, I'm just to send mail.