[email protected]' Asunto 'prova2', Cuenta: '22.214.171.124', Servidor: 'smtp.mydomain.com', Protocolo: SMTP, Respuesta del servidor: '550 5.0.0 Rejected - zen.dnsbl', Puerto: 25, Seguridad (SSL): No, Error de servidor: 550, Número de error: 0x800CCC78
Help! Why can't I send?"
The first thing that springs to mind is that perhasp your ISP is querying blacklists incorrectly. The CBL blacklist recently published some really good info on why you have to be careful to use a DNSBL correctly. If that is what is happening here, then your ISP is checking connections from its own users, and rejecting those connections if your IP address is on a blacklist. That's a problem, because lots of dynamic IP addresses are on "policy" or "dynamic" blacklists. Not because those IP addresses have done bad things, but just because they're not supposed to be sending mail. Your own ISP's mail server is supposed to accept this mail for you, and relay it on. This avoids the blacklisting issue. But if your ISP's mail server is misconfigured, you run into this very problem.
If that's not the issue, then it sounds like you are trying to connect to other peoples' mail servers directly. Simply put, this generally is not allowed. "Policy" or "Dynamic" blacklists (like the Spamhaus PBL) exist to help prevent this practice. Why? Because, back in the old days, spammers regularly tried to send their spam via "desktop mailer" programs that sent directly from dynamic or dialup IP addresses, bypassing their ISP's mail servers, to try to stay under the radar and get away with their spamming. Lots of the spam I got back in the old days was sent via this method, and we ended up effectively prohibiting the sale of "desktop mailer" programs at the company I worked at because they seemed to only be used for these kinds of bad acts.
The solution here is to use your ISP's mail server, not somebody else's mail server. If that doesn't work, contact your ISP and tell them what you're trying to do. Maybe they have to configure their mail server to allow you to send through it using your domain. If they can't help you make it work, then maybe it's time to change ISPs.
Or, consider hosting your domain email with Google Apps. It's free and it basically lets you use your domain with a Gmail-like interface. I use it for my own email and I like it a lot, though certainly Google isn't perfect and I don't agree with everything they do.
Sending mail from your own domain is an important thing to be able to do. I strongly recommend using your own domain, instead of somebody else's domain, as often as possible. Using your own domain, you control your own destiny. If you get locked out of your ISP or webmail account, or even if that company goes under, you don't lose your email address. You can move it to a new ISP or find a new hosting provider. (Anybody remember that lawsuit where the woman sued AOL because they canceled her email account that she was using for her business? If she used her own domain, she wouldn't have lost access to that account; she could have moved it to a new ISP.)