Thunderbird
Thunderbird is a free, open-source email client; this note also records a Flatpak command used to reopen a profile after a version-compatibility error.
Subtopics
Fixing profile version error
Having rebooted my computer after a crash, Thunderbird gave me an error that my user profile was too old for the current version. I was running the latest version. The following command allowed me to reuse my profile instead of having to set Thunderbird up from scratch again.
flatpak run org.mozilla.Thunderbird -p --allow-downgrade