I have several users running Thunderbird as a mail client connecting to an Exchange 2010 server. One user updated from v37 (which was working fine) on Friday before leaving work and on Monday morning can no longer authenticate to the mail server. One user updated this morning and has the same problem. No settings were changed. Those users who have not yet updated are still authenticating without problems. When launched, Thunderbird prompts user to change authentication method. Current IMAP settings are using port 143, STARTTLS and Normal Password. We have tried changing these settings and nothing works. Setting up Thunderbird on a new machine exhibits the same issues. It probes the exchange server and finds the settings but continues to report a problem with the user/password credentials. Same issue with SMTP authentication. Current settings are port 587, STARTTLS, and Kerberos/GSSAPI. Right now, Thunderbird users who updated to V38 are unable to receive or send email via Exchange server.