Frequently people receive e-mail at a "public" mail address, which may even be an alias that forwards the mail on to another address. In reply, you might want to send from a different address. Sometimes, when sending a new (non-reply) message, Microsoft Entourage will naturally send it from the default account unless you remember to change the account manually. You want to be able to send mail automatically from the correct account, especially in business.

By creating a category with the same name as the mail account, and then assigning this category to contacts in your Address Book to whom you want correspondence to be sent only from that account, this script will make it possible. You need to run it from a repeating schedule every one minute so that it catches the message window before you send it, and changes the account while you're writing.

You need to enter the name of each account/category. Each one should be in its own set of quotes, separated by commas, and in the otherAccountNames property list braces, like this: {"Correspondence", "billing@mybusiness.com"}. Some people use the actual e-mail address to name their mail accounts, and some give descriptive names. You must use whatever the names are as they appear in Tools > Accounts > Mail, without the POP, IMAP, or Exchange description.

(*enter account/category names in {},
comma-separated "quoted text" items*)
property otherAccountNames : {}
 
tell application "Microsoft Entourage"

	if otherAccounts = {} then
		beep
		return
	end if

	try
		set frontWin to front window
		if class of frontWin ≠ draft window then
			return -- quit if no new draft window open
		end if
	on error
		return
	end try

	set recipsList to every recipient of frontWin

	set exitRpt to false -- for outer loop
	repeat with i from 1 to count recipsList
		repeat 1 times
			set theRecip to item i of my recipsList
			set eAddress to address of the address of theRecip
			if eAddress ≠ "" then
				try
					set foundItem to item 1 ¬
						of (find eAddress)
				on error -- not a contact
					exit repeat -- 1 times
				end try
			else -- group
				try
					set dName to display name of address ¬
						of theRecip
					set foundItem to group dName 
				on error
					-- text typed in without being group 
					-- or email address
					exit repeat -- 1 times
				end try
			end if
         
			set theCats to category of foundItem
			if theCats = {} then exit repeat
              
			repeat with j from 1 to (count theCats)
				set catname to name of (item j of theCats)
				if {catname} is in otherAccountNames then
					try
						set otherAccount ¬
							to POP account catname
					on error
						try
							set otherAccount ¬
								to IMAP ¬
								account catname
						on error
							try
								set otherAccount ¬
									to ¬
									Exchange ¬
									account ¬
									catname
							on error
								beep 2
								return
							end try
						end try
					end try
                        
					tell frontWin to set its account ¬
						to otherAccount
					set exitRpt to true
					exit repeat
				end if
			end repeat
		end repeat -- 1 times
          
		if exitRpt then exit repeat -- done
	end repeat
     
end tell

Note that the script checks for every recipient. If all you care about is the first To recipient, remove the outer repeat block and exitRpt lines, and do the following instead:

set theRecip to first recipient of frontWin

AppleScript does not have a Next Repeat operator, so note the alternative in the script to effect the same thing by including a repeat 1 times block inside the outer repeat block. That way, when you hit a condition that makes continuation with the current list item unnecessary and undesirable (for example, when no recipient is a contact or group bearing the sought category), the script moves on to the next list item by an exit repeat within the repeat 1 times block, rolling it forward to the next item in the outer repeat loop.

That's why, when there's a condition that requires exiting the outer repeat block, namely having found both a category and the account of the same name, you first have to set the variable exitRpt to true, exit the inner repeat 1 times block, and use the true value of exitRpt after the repeat 1 times block closes as the criterion for exit repeat from the outer block.

Also note that there's no generic account class (although message has an account property), so you need to check for POP account, IMAP account, and Exchange account of the designated name in turn.