There are two reasons why this could be happening. Please read ALL of this Post so you can resolve your issue quickly.
1. If you have recently configured your email program and this is the FIRST time you have tried to use it, you may have not checked the box in your email program that says "My SMTP server Requires Authentication" This is required so that your email program tells your SMTP (sending) server that you are the legitimate sender and that someone is not using your email server to send SPAM or other unauthorized email. If this box is not checked, you will NOT be able to send email. The location of this box varies with the email program you are using, so check the Help file on that program to determine how and where to set it.
2. If your email program was definitely
configured correctly (i.e. this problem just started but before, your
email program was working just fine), your IP may be blocked for sending
of email. Please note! WE DO NOT BLOCK YOUR IP ADDRESS. This is done by a third party. It is very
common in Costa Rica (but not limited to Costa Rica) that SPAMhaus,
SPAMcop or other anti-spam services block the sending of email from a
specific IP address. That would be YOUR IP address.
Follow these steps exactly to see if your IP has been blocked: DO THIS ONLY THE AFFECTED COMPUTER AT THE AFFECTED LOCATION! It cannot be done at another location!
Go to http://www.whatismyip.com This will give you your IP address
that has been assigned to you by the company that provides your Internet
Service. That is NOT US. We provide your HOSTING Service!
B. Record the IP address that is given to you. Write it down.
(Regardless of what you THINK your IP address is, THIS is the IP address the world thinks you are using. Why a difference? Usage of firewall, proxy server and other reasons can explain this).
Now go to: http://www.dnsbl.com/. and enter the IP address where it
shows: "blacklist lookup". Please note there are several companies that
you can use for checking if an IP is on a Blacklist.
D. On the resulting page, scroll down to see if your IP has been blacklisted.
If it is, you must then contact your local ISP (ICE, RACSA, AMNET, CABLETICA, etc. in Costa Rica). If you are not in Costa Rica, then contact the company from which you purchase your Internet service. Please note that your Internet service provider will NOT know of this block! You must tell them and they will then be able to fix the issue.
NOTE: If you use a dial up (modem) connection, your IP changes every time you dial in to your account. Therefore, just hang up and login again and the next time, you may (but not always) get a different IP address that is not blocked.
you can re-configure your email program and this might solve the
problem. Normally, we recommend using SMTP port 25. Try changing this
to port 2525. This is a workaround and not a solution. Port 2525 is not available on all servers.
If your computer(s) are not FULLY protected against viruses, and you are not using a virus protection program that is CURRENT, you may have a virus or Trojan horse on that PC that is sending SPAM without your knowledge! If this is the case, you will have to remove all viruses before your Internet Service Provider can help you. This is VERY common for companies or people who do not have anti-virus protection installed on every computer!