Thunderbird email client not sending email with Exchange Online

Overview

Thunderbird email client is not sending email with Exchange Online - typically after an upgrade to Thunderbird client.  Resolution is to disable IPv6.

Procedures

1. In Thunderbird, click on the Settings option in the program's main drop-down menu.  Then go to  General --> Config Editor --> Show All
2. In the search Find in Settings box, type network.dns.dis  -- press Enter
3. The first option should be network.dns.disableIPv6 -- select this and toggle to change it from false to true
4. Close and reopen Thunderbird

NOTE:  This configuration setting is global for all accounts configured in Thunderbird and is effective for Exchange Online, Office 365.  However, if a different account requires IPv6 it will cause an issue with that account.

Alternative if the above does not work or you do not want to globally disable IPv6 for Thunderbird:

1. If you made the change above to IPv6 you must first switch that back to false
2. In Thunderbird, click on the Settings option in the program's main drop-down menu.  Then go to  General --> Config Editor --> Show All
3. In the search Find in Settings box search for ipv4o -- press Enter
4. Add outlook.office365.com to the text box next to "network.dns.ipv4OnlyDomains".

References

None at this time.

Last Reviewed:   30-Dec-2022