Few things are obvious: (1) sender must be able to use simple _attach file_ feature to send a file, without extra skills; (2) receiver should be able to receive this file and open it as part of letter; without extra skills. If you want to create additional service, you can (by request) retract attachments on the server, place them on the web (you can virus - scan them if you want and add virus warning) and include html reference into the mail itself; but it cause a lot of problems if customer uses something other that openlook - express. For some cases it looks reasonable; for opthers, IMAP can solve the problem. And (of course) you need _smart_ anty-spam filters which can be turn on / off by the customer (and can have ON default). My friends filter out SPAM by finding the same messages send to the hundred of the custoimers at onse - this simple method drop the SPAM percent dramatically (just again, it's important to allow receiving spam - it can be something except SPAM, so they provide some way to look through removed messages, as I know). Anyway, SPAm p[roblem and PARSEL problem are different ones.