First of all, AcyMailing always uses a queue to send your e-mails.
So, if the message can not be sent immediately (time out, limitation of a shared hosting environment, memory limitation, etc.), AcyMailing will keep it in the queue so you will be able to send it later.
If you are reading this article because you saw a lot of errors during the send process and the process stopped, don't panic! You will be able to easily resume your send process and you won't loose any message.