Can't I download the software after closing imessage?

No, you can download it by pressing, but a message box will pop up to remind you that closing imessage has no effect, but from time to time you will encounter a pop-up prompt automatically whether to activate iMessage.

iMessage is an instant messaging software launched by Apple, which can send short messages, videos, etc. It has very high security. Different from the SMS/MMS service of operators, users only need data support through WiFi or cellular data network to complete communication.

with iMessage, you can send and receive messages to and from anyone who uses iPad, iPhone, iPod touch or Mac users who run Mountain Lion, and you can also send photos, videos, location information and contact information.

if you have more than one Apple device, iMessage can keep continuous conversations on all devices. Moreover, you can send text messages, photos and videos to other mobile phones through the cellular network. You can even ask Siri to send you a text message. Just say "Tell Xiaoli I'm on my way" and Siri will help you write and send messages.

difference from SMS:

iMessage information is text, photos or videos that you send to another iPhone, iPad, iPod touch or Mac via Wi-Fi or cellular data network. This information is usually encrypted and appears in blue text bubbles. To turn iMessage information on or off, go to Settings > "information"

if you don't use iMessage information, you can use SMS/MMS. These messages are texts and photos that you send to other phones or other iPhone, iPads or iPod touch. SMS/MMS messages are not encrypted and appear in green text bubbles on the device.