Incorrect sender names suddenly attaching to emails today

A weird thing has started happening to my Thunderbird inbox in the last couple of days. Emails are showing as from a totally different sender. For example, a Google search on Sondheim has been "sent" by EDF (my electricity supplier) and some Groupon emails are showing as coming from people I correspond with. Also a Keto bread recipe from a low carb group I belong to has been "sent" by one of my work clients! More weirdly, I copied them into a subfolder on my inbox and they then appeared correctly. I have tried repairing the folder but it made no difference.