I thought I'd have a quick look at what SIZE values SMTP servers
are advertising. Censys have a EHLO banner grab for all of the IPv4
address space from both 2016 and 2018. In 2016, they recorded 4352858
EHLOs, of which 3950125 had a SIZE response. In 2018 the figures
were 6077111 EHLOs with 5535203 including size.
I plotted a histogram of the resulting sizes here:
http://www.maths.tcd.ie/~dwmalone/smtpsize.png
Some of the SIZE responses are 0 (meaning no limit, according to
RFC 1870) or blank (meaning no information at this time). I've
mapped the blanks all to the size zero bucket to be conservative.
The no-limit SIZEs have been mapped to a very large size.
It seems like sizes of 10GB, 20GB and 50GB are common in both 2016
and 2016. The median is more than 20GB both times.
David.
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