''<p>cmorrisonaz [[#question-1357557|said]]</p>'' <blockquote>Thunderbird, How could you make the upgrade from 78.14 through the release channel to 91.3.0 without telling your users that it won't be backwards compatible, that reverting to 78 isn't possible? </blockquote> V78 and to some extent 68 all had the same restrictions. https://support.mozilla.org/en-US/kb/dedicated-profile-thunderbird-installation <blockquote> I cannot send or receive email with the upgrade to 91.3.0, and it appears I'm not the only one having this problem. </blockquote> This is most usually caused by anti virus/ internet security suite software post update. Use the instructions in the link to restart [https://support.microsoft.com/en-us/help/12376/windows-10-start-your-pc-in-safe-mode Windows 10] in safe mode with networking to see if that is your problem as well. <blockquote> Attempting to install v 78 on a different computer and importing the original profiles fails. </blockquote> See the above link for details on the profile per install "feature" <blockquote> None of the fixes I was able to find online resolves the problem. </blockquote> Did you look at your anti virus? The most likely cause of a connection issue or send receive problems in using an email client. It even exceeds the issues from incorrect setups in my opinion. <blockquote> Please send out a patch to let users revert to 78.14 so I can send and receive my mail again. </blockquote> Not going to happen, so lets fix your issues with the latest release.