Sending (PHB.274): (emphasis mine)
Casting Time: 1 action […]
Duration: 1 round […]
You send a short Message of twenty-five words or less to a creature with you are familiar. The creature hears the Message in its mind, recognizes you as the sender if it knows you, and can answer in a like manner immediately. The spell enables creatures with Intelligence scores of at least 1 to understand the meaning of your Message. […]
What constitutes immediately? The spell has a casting time of 1 action, and a duration of 1 round. Does the 1 round make up how long the caster has to send the message after the spell has been cast? Does that same time restriction apply to the receiver? Do both messages have to fit within the same round? Or does the 1 round represent the time it takes for the spell to arrive at the receiver?