The message was blocked by Outgoing Mail Control, when will it be possible to send messages again?
Outgoing Mail Control checks mail history on attempt to send an email. If messages count sent in the last hour equals to limit, the message is blocked.
Note: Outgoing Mail Control counts only messages which were sucessfully sent, it does not count blocked attempts to send messages.
Mailbox is limited by 2 messages per hour. The first email was sent at 10:40, the next one was sent at 10:52. If you try to send another message at 11:05, you will be blocked. It will be possible to send a message after 11:40, and another one - after 11:52.