Creepy privacy
When authorizing Airmail to access e.g. your gmail account, its not just the app that have access but also Airmail servers. According to their support the access will not be used if notifications is disabled, but there is no way to disallow Airmails servers/staff the access (i.e. one cannot distinguis between giving the app access to your email and Airmails servers access).
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Airmail - Your Mail With You, v1.6.4