I am considering moving from SmarterMail to hMail because SmarterMail requires almost 270Mb of RAM and I only have one domain with about 20 email accounts. Can anyone tell me how much RAM hMail requires and what drives hMail's need for RAM?

Hardly a feature request.

I reckon the answer is 'depends'

depends on which database you use.
depends on whether or not you use hmailserver's inline AV scanning
depends on how your users connect.

If you use MySQL and don't use ClamWIN, all users are POP3 then less than that I reckon.

Thank you for your reply even though I posted into the wrong category.
1) I prefer to use SQL Server.
2) I will use hMail's inline AV scanning.
3) I create email accounts to provide "From:" detail for out-going email messages my web-based application generates. The application really only needs an SMTP service. Users never logon and don't need POP3 as every email account forwards incoming messages to the user associated with the account.

My application regsiters volunteers, organizes committees and manages teams for schools. Each school has its own email account. Volunteers exchange email within their community and the "From:" portion of the header is "SchoolName"@TheSchoolVolunteer.org. Outgoing messages insert the originating volunteer's email address in the "Reply-to:" portion of the header.

So how to I estimate my RAM requirements?

How many messages per day?
Are they spaced out or are there bottlenecks?

The bigger issue for you is the memory usage of MS SQL Server. SQL Server CE that ships with hMailserver isn't brilliant, but an external MS SQL server will *ALWAYS* quarantine more memory than it needs unless the instance is specifically limited. Check the documentation for your choice of SQL.

MS SQL CE isn't designed for serious use, and it ships with hMailserver only due to the requirement that hMailserver 5.0 >> 5.3 wasn't open source and therefore couldn't ship with MySQL.

If it helps as a basis of comparison, I run hMailserver that handles 23 different domains, approximately 160 mail accounts spread between them, and average of 100 users connected to their mailboxes at one one time and everyone connecting via IMAP protocol.
The database is SQLExpress, and the system processes an average of around 2,500 emails a day (in and out).

hMailserver generally needs around 150MB of RAM, with the SQL process taking around 650MB.

hmail with mssql ce is going to use very little ram but would only be useful for light use. on other hand if you opt for mysql instead then hmail won't use any more but you tell mysql how much to use with my.ini. You could end up with 15MB used or 1GB.

Keep in mind the amount of ram hmail uses depends on your usage & settings. IMAP uses the most so the more IMAP users you have the more & the larger the messages they open the more etc. On other hand if all your users were pop-only hmail would use very little. Your caching settings & index settings & any vacation responses you have enabled, scripts you have set etc etc all add more ram use. (hmail ram use grows over time due to thins stored in mem vs db so that is why it'll be the least after 1st starting vs after months of running)
Bill