Some clients(qtox antox isotoxin toxy) implement pseudo-offline messaging. Pseudo-offline messaging essentially schedules a message to be sent to a friend the next time you are both online. While this can be convenient at times, it still requires both clients to be online at the same time for one to message the other. Fully offline, asynchronous messages would not carry this requirement.

Pseudo-offline messages work similarly to the "Pounce" feature some people may be familiar with from Pidgin. More about Pounce.

1. Type a message intended for an offline recipient. This message is saved and stored on your client for later delivery, when the friend becomes available.
2. Wait, online, for your friend to become available. Do other stuff, read a book, play a video game, but try not to leave your computer unattended.
3. When the friend comes online, the message will be sent to that friend immediately.

Some people have reported setting up computers they trust to relay messages to them when they are offline, often using single-board computers and ratox or toxbot to set up the relay. It is up to you to decide if these are appropriate for your needs. Each of them could potentially carry

1. Astonex's Tox Message Relay Relays messages on behalf of a another Tox user. Only supports one client at a time and requires you to run it on a computer which is always online.
2. Toxmail's email to Tox Messaging Bridge Lets you create or authorize “Supernodes” to deliver messages on your behalf when you're offline, but compromises on decentralization.
3. Scripting an interface to Tox using Ratox could allow you to home-brew a message relay Ratox AutoAnswer Nugget Ratox AutoApprove Nugget Ratox Echo Script

Set of proposed requirements for offline Messaging in Tox

Proposal for offline messages from GSOC (not accepted: GSoC 2015 accepted proposals)

Github issue #186 Offline Messaging

Github issue #870 Recieving messages when offline/online