This makes messaging offline players pretty simple, without all the extra bloat of a mail mod. If the bloat isn't a problem I would reccomend a mail mod and not this mod, as it is really useful to see your old messages and keep up with conversations better and answer messages when you want to!
Although this adds a good way to message offline players, it isn't practical: You're only going to see the message once, so you need to either reply quickly or copy the message down and reply later - not convienent. Plus, getting spammed by messages when you join the game is not often a lot of fun! And if the player is going to respond to them is a whole different matter. So, this isn't even close to the perfect solution, but it will do the job for some servers.
I'm only recommending this because the description says Sure, some servers have an email/mailbox system, but for many, such systems are overly complex. That's why I made tell. - but if you can spare the space for a mail mod, I wouldn't recommend this.