What does it mean when an iMessage is sent as a non iMessage just a regular text message to another iphone?

What does it mean when an iMessage is sent as a non iMessage just a regular text message to another iphone? - 1

It means iMessage isn't available, this can be down to:

- No available Internet on either your end or their end (including 3G/4G)
- They/you have iMessage turned off
- their phone is off or on Airplane mode
- they've blocked your email used for iMessage (so it sends via your phone number instead)

Either the iMessage didn't send for whatever reason or the person disabled iMessage