I'm looking for reliable statistics on the breakdown of number of
email accounts per domain in the US, especially for non-business
internet domains.  For example, in the US, there are XXX domains with 1-5 email
addresses, XXX domains with 6-50 email addresses, XXX domains with
51-200 email addresses, etc, etc.

Thanks!
Jeremy

this is impossible to tell, unless you have a spammers database (then
you could only estimate)

The above comment is correct - there is no easy way to obtain this
information on your own.

I would try contacting a random assortment of web/email hosting
companies and asking them for this information. If you can cultivate
enough data from enough companies, you would have a good sample to
work towards your average. Be forewarned that many companies will
probably not release this data.

Companies like www.netcraft.com and www.isc.org collect statistics
about internet usage and growth - they may have a product or web page
for you.

Depending on your use, you may find a number of companies willing to
release this to you.

Per the above user's advice - you could try a spammer's database - set
up a count in your software to sort by domain name and make a count
per each domain - but be aware of two issues you may run into. First,
you want a database of verified emails, not a collection of "guesses -
some spam lists are not verified and often have common email names
listed for each domain whether or not they actually exist (this would
inflate your numbers). Verified lists will not include every possible
name for each domain and will therefore be lower then an actual count.

Good luck!