If a contact is offline, then your instant message won’t be delivered to them immediately. However, Skype saves the message for 30 days after you send it, and delivers it as soon as both you and the recipient are online at the same time.

Because instant messages are saved by Skype, even if you sign in on a different computer, the message will still be delivered as soon as you and the recipient are online. All messages sent and received on one computer in the last 30 days are copied to the other, allowing you to synchronise your instant message history between computers.

Skype stores saved messages in encrypted format, and deletes them after 30 days. For more information on how Skype manages personal data, please refer to section 13 of the Skype Privacy Policy.