Tip for users having problems with 1.6.5 and 1.6.6 ..... undocumented. Go to settings of all your gmail accounts > server settings > refresh tokens. Then go to settings on each gmail account and click Reset message cache. Seems to fix the horrible problems I have been experiencing. Will test over next 24 hours and confirm. Thanks to Airmail support for these suggestions but this should be documented.
MK-SouthAfrica about Airmail - Your Mail With You, v1.6.6