To elaborate on the previous post:
You are not blocked from sending, but some of your recipients may be blocked from receiving (a subtle but important difference).
Spamhaus (and others) run blacklists (not a block lists) of mail *servers* that have been reported as sending spam (as defined by that BL's individual criteria).
Your recipient's Internet Service Provider (ISP) can subscribe to one or more blacklists, and at their option either flag mail as suspected spam if it originates from a mailserver on the BL, or reject mail outright if from a BL'd mail server address. Note: that it is each recipient's own ISP that makes the choice to block or not for their accounts, it is not the BL operator somehow blocking mail on the Internet.
Hopefully, the customer of that ISP (your intended receiver) has some control over the spam filtering on their mail account, and can choose whether flagging or blocking is used. Your recipient may also have the ability to add your email address and/or your mail server's IP address to a personal "whitelist" or "Friends list" to ensure your email gets through to them.
Why are YOU blacklisted? Simple answer - YOU are not - it's not personal. The Internet email system has no way of knowing WHO sent an email, only that the email originated at the IP address of a specific mail SERVER - which is probably your ISP's mailserver. If someone, anyone, using the same ISP as you breaks the "rules" of the BL, your ISP's mail server IP address will be enrolled in the BL, and ALL mail from that IP will be affected, including yours. If the rulebreaking continues, it is likely to remain BL'd forever. If it was a one-time incident, there may be an appeals process or an automatic process that will get your ISP off the BL in days. Your ISP must initiate this.
If your ISP does not get the problem solved, your only option is to change your mail services to a totally different ISP that uses different mail servers.
One additional thing about broadband accounts and mailservers:
If you have broadband (Cable or DSL) service, and you run your own mailserver or mailserver-like programs, you may find yourself blocked out of hand -- this is because some BL's automatically consider any mailservers within a "dial-up" or home broadband block of IP addresses to be rogue mailservers (spammers or zombied machines) or virii.
Sometimes, commerical DSL/Cable netblocks get improperly identified as "dial-up". The key differentiator is whether your broadband connection has a static or a dynamic address. If you have a dynamic IP address, there is probably nothing you can do about being blocked by this method and your should use your ISP's mailserver. If you have a legitimate static (or "business" ) IP address, your ISP can probably appeal to get that block of addresses removed from the BL.
Thanks
Trevor
CanadaRAM.com