If a message is time sensitive, an automatic expiry time can be set so that if the message isn't delivered within a certain time, it won't be retried. This is called a validity period and can be set in the options section.
The validity period is measured in hourly increments, so if you will be available for a meeting in two hours but not for the rest of the day, the validity period can be set for two hours.
A message can then be sent to those who wish to meet with you at that time. If their handset isn't available to take receipt of that message until after the two hour window, when having a meeting won't be possible, the message will expire.
