[joosters]

If all the API calls create a mailbox on demand, and never return a "mailbox doesn't exist" error, then the GET doesn't make any observable changes - it's as if the mailbox has always existed.

[Drakim]

Provided that there aren't any other consequences, such as emails not being delivered before you do that GET.

[cryptonector]

Idempotent != no side effects.

Idempotent == if you do it more than once, you get the same result.