Devised a solution that works. Instead of a single line, selected all messages in the folder and looped through them, grabbing the account name as text and doing the compare and move.
tell application "Microsoft Outlook"
set topFolder to mail folder "AT&T" of on my computer
set destFolder to folder "Deleted Items" of topFolder
set srcFolder to mail folder "Deleted Items" of on my computer
set selectedMessages to messages of srcFolder
repeat with theMessages in selectedMessages
set thisAccount to account of theMessages
if (name of thisAccount as text is "Att") then
if (is read of theMessages is false) then
set theMessages's is read to true
end if
move theMessages to destFolder
end if
end repeat
end tell