Thunderbird

• Menu → Preferences → Preferences → Advanced → General → Config Editor
• Set the preference called mail.wrap_long_lines to false
• Set mailnews.wraplength to 0
• Restart Thunderbird.

• Rebuild global database – delete global-messages-db.sqlite in profile folder
• Check activities: Tools → Activity manager

You may experience a lot of send/receive traffic in the background.

In this case do a complete sync in Menu > File > Offline > Download/sync now

If you don't want to sync all of your account data, you can change your account settings to sync only the last x days of mail.