The number of offline messages is capped at 15 messages for Basic & Plus members, 80 for Premium members, and 160 for Premium Plus members.
If you send a message to an offline user, you can basically never be sure that they will receive it... That's why so many people prefer to send notecards instead.
I think that would be nice if, when sending a message to someone offline and this message is capped, receive a notification to let us know that the recipient will never read it, so in this case we know that we should send a notecard instead.