Common Thunderbird Email Errors

Mozilla Thunderbird is a solid free email client, and we see a good number of Ultra Web Hosting customers using it. When things go wrong, Thunderbird's error messages are usually more descriptive than Outlook's, which makes troubleshooting easier. This guide covers the most common Thunderbird errors and how to fix them.

01. Correct Settings for Thunderbird

Thunderbird has an auto-configuration feature that tries to detect settings automatically. For Ultra Web Hosting accounts, auto-detection sometimes picks the wrong ports or encryption. If auto-config fails, set it up manually:

Manual IMAP Configuration

Incoming:
  Protocol: IMAP
  Server: yourserver.ultrawebhosting.com
  Port: 993
  Connection Security: SSL/TLS
  Authentication: Normal password

Outgoing:
  Server: yourserver.ultrawebhosting.com
  Port: 465
  Connection Security: SSL/TLS
  Authentication: Normal password

Username (both): your full email address

Manual POP3 Configuration

Incoming:
  Protocol: POP3
  Server: yourserver.ultrawebhosting.com
  Port: 995
  Connection Security: SSL/TLS
  Authentication: Normal password

Outgoing:
  (same as IMAP above)

02. Connection Errors

"The connection to the server was refused" or "Connection refused"

Thunderbird can't establish a connection on the specified port. Causes:

• Wrong port number - use 993 for IMAP, 995 for POP3, 465 for SMTP. Double-check in Account Settings > Server Settings (incoming) and Account Settings > Outgoing Server (SMTP)
• IP blocked by firewall - too many wrong password attempts triggers a block. Unblock yourself at my.ultrawebhosting.com
• Network blocking ports - corporate or hotel Wi-Fi often blocks email ports. Try from a different network

"The connection to the server timed out"

Thunderbird reached the server but didn't get a response in time. This is different from "refused" because the server was reachable but slow to respond.

• Increase timeout - go to Edit > Preferences > Config Editor (about:config), search for mailnews.tcptimeout, and increase it from the default 60 to 120
• Server under load - if it's intermittent, the server may be handling heavy traffic. Try again in a few minutes
• DNS issue - try using the server's IP address instead of hostname as a test

"Unable to connect to host" with a specific hostname

DNS can't resolve the server address. Verify the hostname is spelled correctly. Try pinging it from a terminal: ping web150.ultrawebhosting.com (use your actual server). If ping fails, there's a DNS issue on your network.

03. Authentication and Login Errors

"Login to server failed" or "Authentication failed"

This is the most common Thunderbird error. The server rejected your login credentials.

• Verify the username - it must be your full email address (info@yourdomain.com), not just info
• Verify the password - log into cPanel > Email Accounts and reset the password for this address. Then update it in Thunderbird
• Check authentication method - in Account Settings > Server Settings, make sure "Authentication method" is set to "Normal password." Not "OAuth2," "NTLM," or "Kerberos"
• Clear saved password - go to Edit > Preferences > Privacy & Security > Saved Passwords (or Passwords > Saved Logins in newer versions). Remove the entry for your mail server, then let Thunderbird re-prompt you

"IMAP server closed the connection" immediately after login

This can mean the IMAP service is restarting, the account is over its connection limit, or there's an issue with the specific mailbox. Try waiting 30 seconds and reconnecting. If it persists, check if another device or program has too many simultaneous IMAP connections open (some mobile apps are aggressive about holding connections).

04. Sending Errors

"Sending of the message failed" - Authentication Required

The SMTP server requires you to log in before sending. Go to Account Settings > Outgoing Server (SMTP), select your server, click Edit, and verify:

• Security and Authentication > Connection Security: SSL/TLS
• Port: 465
• Authentication method: Normal password
• User Name: your full email address

"Sending of the message failed" - Relay Not Permitted

The server rejected the message because it doesn't think you're authorized to send through it. Same fix as above: make sure SMTP authentication is configured with your email address and password.

"Sending of the message failed" - Timeout

The SMTP connection timed out. This happens with large attachments on slow connections, or when port 465 is blocked. Try port 587 with STARTTLS as an alternative. If sending small emails also fails, check if your IP is blocked.

"Message could not be sent because connecting to SMTP server failed"

Thunderbird can't reach the SMTP server at all. Check that the outgoing server hostname is correct, the port is 465, and your IP isn't blocked. Test by switching to your phone's hotspot to rule out a network issue.

05. SSL and Certificate Errors

"Add Security Exception" Dialog

Thunderbird is warning that the SSL certificate doesn't match the server you're connecting to. This happens when you use mail.yourdomain.com but the certificate is issued for webXXX.ultrawebhosting.com.

Proper fix: Change the server to your actual server hostname (webXXX.ultrawebhosting.com) in both incoming and outgoing settings. The certificate will match and you won't see this warning again.

Quick fix: Click "Confirm Security Exception" to accept the certificate. This works but you'll need to re-accept if the certificate renews.

"The certificate is not trusted because it is self-signed"

Same cause as above. The server's AutoSSL certificate is for the server hostname, not your domain. Use the server hostname instead of mail.yourdomain.com.

"Peer's certificate has an invalid signature" or "SEC_ERROR_BAD_SIGNATURE"

This is rarer and usually means something is intercepting the SSL connection (corporate firewall, antivirus, or a proxy). Disable antivirus SSL/TLS scanning to test. If you're on a corporate network, check with your IT department about SSL inspection.

06. Sync, Folder, and Display Issues

Sent Messages Don't Appear in Sent Folder

Thunderbird has separate settings for where to store sent mail. Go to Account Settings > Copies & Folders > "Place a copy in" and make sure "Sent" folder is selected under "Sent Folder on" your IMAP account (not "Local Folders").

Folders Missing or Not Syncing

Right-click your account in the folder pane and select "Subscribe." This shows all available IMAP folders on the server. Check the ones you want visible in Thunderbird. Also try right-clicking the account and selecting "Get Messages" or "Compact Folders" to force a sync.

Email Shows as Plain Text Instead of HTML

Go to View > Message Body As > Original HTML. If it's set to "Plain Text," Thunderbird strips all formatting from incoming messages.

Duplicate Messages

Usually caused by having the same account added as both POP3 and IMAP, or the account is configured in Thunderbird twice. Check Account Settings and remove any duplicate entries. If you recently switched from POP3 to IMAP, Thunderbird may have downloaded messages to local storage and also synced them from IMAP.

"Folder Busy" or "The current command did not succeed"

Thunderbird is trying to perform multiple operations on the same folder simultaneously. Wait a moment and try again. If it keeps happening, go to Account Settings > Server Settings > Advanced and reduce "Maximum number of server connections to cache" to 3 or 4. Also make sure "Clean up (Expunge) Inbox on Exit" is enabled to keep the mailbox tidy.

07. General Fixes

If the specific error fix above didn't solve it, try these in order:

1. Remove and re-add the account - go to Account Settings, select the account, click "Account Actions" > "Remove Account." Then add it fresh with the correct settings from Section 1
2. Clear Thunderbird's password cache - Edit > Preferences > Privacy & Security > Saved Passwords > remove entries for your mail server
3. Test with Webmail - log into Webmail (cPanel > Email Accounts > Check Email). If Webmail works, the server is fine and the problem is Thunderbird-side
4. Start Thunderbird in Safe Mode - hold Shift while launching Thunderbird. Safe Mode disables addons that might be interfering
5. Try a fresh Thunderbird profile - launch Thunderbird with the -P flag to open the Profile Manager, create a new profile, and set up your account there. This rules out profile corruption

Quick Recap: Fix Thunderbird Errors

If you only do 5 things from this guide, do these:

1. Use manual config, not auto-detect - Thunderbird's auto-config often picks wrong settings for cPanel hosting
2. Use your server hostname - webXXX.ultrawebhosting.com for both incoming and outgoing to avoid SSL errors
3. Authentication must be "Normal password" - not OAuth2 or GSSAPI
4. Check if your IP is blocked - unblock at my.ultrawebhosting.com after failed login attempts
5. Test with Webmail - if Webmail works, the server is fine and the issue is in Thunderbird's config

Last updated March 2026 · Browse all Email articles · See also: Email Troubleshooting Guide | Email Authentication Guide