Fix Thunderbird when receiving works but sending mail fails
What to do first
Repair only the outgoing SMTP path without changing incoming settings or deleting local mail.
This is a general troubleshooting guide, not an announcement of a current outage.
Affected products and symptoms
- Product
- Mozilla Thunderbird
- Versions and conditions
- Thunderbird Desktop on Windows, macOS, and Linux
- Error codes and identifiers
- SMTP
Symptoms
- Mail downloads, but sending fails
- SMTP authentication, connection refused, or timeout appears
Causes and conditions
Check the sender-to-SMTP association, port, encryption, authentication, saved credentials, and VPN or security filtering in that order.
Before you start
- Record the complete sending error, time, and recipient
- Open the mail provider’s official SMTP page and confirm webmail access
Precautions
- Immediately re-enable security software after a brief diagnostic test
- Never post SMTP passwords or full account details publicly
Step-by-step instructions
Send to the same recipient from webmail to rule out an account suspension or recipient rejection. Follow company policy before testing another network.
Receiving success does not validate SMTP because incoming and outgoing servers have separate settings. Preserve incoming settings and local messages; record the server and error number.
Open Menu > Account Settings > Outgoing Server (SMTP), then compare hostname, port, connection security, authentication method, and username with the provider documentation.
Do not guess common ports. Port 587 with STARTTLS and 465 with SSL/TLS are different combinations, and servers limited to obsolete TLS can be rejected by current Thunderbird.
In Account Settings, select the mail account and check Outgoing Server (SMTP) at the bottom. It must point to the server authorized for that address.
With multiple accounts, another provider’s SMTP server may authenticate but reject the From address. Select the right entry instead of deleting all existing entries.
After an update, check whether a VPN, proxy, antivirus mail scanner, or firewall stopped trusting the new Thunderbird build. Allow it using administrator or vendor guidance.
If configuration is correct but authentication fails, open Settings > Privacy & Security > Saved Passwords, remove only the relevant SMTP credential, and enter the correct password on the next send.
Send one short, attachment-free message to yourself. Confirm it in Sent and in webmail.
Repeated retries can trigger account lockout. Stop after continuing authentication failures and ask the provider whether 2FA or an app password is required.
Check the result
- An attachment-free test reaches the mailbox and appears in Sent
- Sending from the same identity still works after restart
If the problem continues
- Use webmail and give the provider the full error text
- Ask the organization administrator about permitted SMTP authentication and device rules
Scope of this guide
Troubleshooting guide — Follow Mozilla’s official sending guide: capture the error, then isolate SMTP configuration, account association, network filtering, and credentials.
Frequently asked questions
If receiving works, must the password be correct?
Not necessarily. SMTP can use a separate stored credential and authentication method.
Should I switch to port 25?
No. ISPs commonly block it and its encryption pairing differs. Use the provider’s documented port, commonly 587 or 465.
Official sources and dates
Source publication or resolution date: Not specified. Sources checked: 2026-09-05. The check date is not the date the problem first occurred. Interface labels can vary between versions and display languages.
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