The error you’re encountering:
Error Number: 0x800CCC0B
Protocol: SMTP, Port: 25, Secure(SSL): No
is typically associated with “Server or maildrop is busy” in Microsoft Outlook or Outlook Express. This error occurs when the mail client is unable to connect to the mail server properly. Here’s a breakdown of what might be causing it and how to fix it:
Fix: Restart Outlook.
Fully close the application (make sure it’s not running in the system tray).
Reopen Outlook and try sending the email again.
Many ISPs block outgoing SMTP port 25 to prevent spam.
Fix: Try using port 587 with STARTTLS or port 465 with SSL instead.
In your email client settings:
Outgoing server (SMTP): 587
Encryption method: STARTTLS or TLS
Alternatively: 465 with SSL/TLS
If your server is receiving too many connections (especially from the same IP), it might throttle or temporarily reject them.
Fix:
Wait a few minutes and try again.
Reduce the frequency of send/receive requests in Outlook.
Outlook may hang if there are problematic or large emails in the Outbox.
Fix:
Go to the Outbox and delete or move the stuck message.
Then try sending a test message.
Fix: Double-check your SMTP settings:
Server name (e.g., mail.yourdomain.com)
Authentication enabled (Use same settings as my incoming mail server)
Username: full email address
Password: correct and updated
Antivirus software or firewalls might block SMTP traffic.
Fix:
Temporarily disable antivirus email scanning.
Add exceptions for your email client and SMTP ports in your firewall.
You can test if the SMTP server is reachable by using telnet:
telnet yourdomain.com 25
If you get a response like:
220 yourdomain.com ESMTP Exim...
It means the server is online. If not, port 25 might be blocked.
If port 25 is blocked, try this in your email client settings:
SMTP Port: 587
Encryption: STARTTLS
Or SMTP Port: 465
Encryption: SSL