A validity period is how long a message stays eligible for delivery. If the handset is unreachable until the period ends, the message expires undelivered instead of arriving late.
Why messages need an expiry
Phones are off, out of coverage or in airplane mode more often than senders assume. Without an expiry, a one-time passcode sent at noon can land at 7 p.m., long after the code stopped working, and a "your table is ready" text can arrive after the customer has gone home. A validity period turns "deliver eventually" into "deliver by then or not at all", which is the right behaviour for anything time-bound.
How it is set
In RCS Business Messaging an agent message carries an expiry, either as an absolute time or as a time-to-live from when it is sent; a message the platform cannot deliver before that moment is revoked and the sender receives no delivery receipt for it. SMS has a similar validity field on most carrier routes. A platform exposes both under one setting so the RCS message and its SMS fallback expire together.
Choosing a value
Minutes for passcodes and fraud alerts, hours for same-day appointment reminders, a day or so for promotions, and nothing shorter than the window in which the message is still useful. Expired messages show as a distinct outcome in SimplyRCS analytics so they are not mistaken for delivery failures.