"550 connection denied after dictionary attack Port 25 Security (SSL) Server error 550. Error number 0x800CCC79"
If you are seeing this or similar in your mail client when trying to send email, it can be from several things.
THIS IS THE MOST COMMON REASON!
1) You are trying to send email using the SMTP server without first POP'ing the server to authenticate, or without authenticating at all.
Solution: THIS IS THE MOST COMMON ANSWER!
Try checking the box to SMTP AUTH. In Outlook express this is called:
Servers > Outgoing mail server > My server requires authentication.
In Mac and other platforms, this option to authenticate is usually located on the dialog where you configure your outbound / SMTP server. It is usually a checkbox.
The other option is to make sure you check your POP addresses before attempting to send email that way you have already been authenticated. (ala click send/receive and login to the POP server first, making sure you are not sending first, but this is not recommended probably not even for advanced users because it is inconvenient.
Your authentication expires after 30 minutes so you will have to re-authenticate if you have been idle for this long in order to send mail. If you configure you mail program to check for email at least once every 30 minutes minutes then you should remain authenticated and might not need to check the box, assuming your computer is on 24 hours a day and is never offline.
But for best practices, always check the "my server requires authentication" box. It's foolproof.
Also of notable interest:
Checking the inbox hyperactively say every 5 minutes or less sometimes can lead to the server banning your IP address because it assumes that you are trying to do something bad by connecting to the server so frequently. Turn the frequency down a little bit to prevent this from happening.
2) Your email program has been incorrectly configured and you have attempted to login too many times in a row, in a short duration of time, with the wrong password or username. The server probably has temporarily blocked access from your IP address.
Solution: make sure to remove any misconfigured accounts from your email reader and the remaining accounts that do connect should have the correct password. turn down the frequency of your checks for email to the server to 6 minutes or mare.
If this does not solve your problem, we suggest running an up to date anti-virus program as it is possible that your PC has been blocked by the mail server for sending too many incorrect SMTP requests.
3) You have four or more items in your outbox that match certain filtering criteria. Namely, four emails improperly addressed. When this happens and the SMTP connection cannot be completed the server assumes you are a spammer trying to send arbitrary emails to the general Internet population. It then blocks you.
Solution: Purge your local outbox after verifying your To: line is incorrect, wait a few minutes (close your email reader) and try again.