Ran some stats on my mails of the last 4 years, here are the daily characteristics:

N = 24k Min = 1 Max = 211 Median = 11 Avg = 16.023907 Stddev = 18.312062

A lot of them are actually chat messages through DeltaChat so not representative of usual mail activity. When I remove them I get this:

N = 16k Min = 1 Max = 56 Median = 10 Avg = 11.378933 Stddev = 8.1572529