I don't see this previously suggested so I thought I'd start a new thread.
I suggest the ability to add a short message to direct trades. Not a full mail-length, but a few characters long.
A few times I've sent a trade to a person and have had to send a separate mail to explain why I've sent it.
An example-
Most of us have friends or guildies that have a minus production of water, for example. It might be because of upgrading, poor layout, short-term over demand- whatever. It's not a problem in guild as they can ask and we send, but if they are not online to request it, we can't know to send. It would be nice to be able to send 20k water with a note saying something like "I know you're often short of water, please accept this if you need it or decline if you don't".
Another example-
If it's a friends' or guildies' birthday and you want to send them a gift in game with a message "Happy Birthday!"
I won't bore you with other examples, but hopefully you can see what I mean