A Mailock message can only be opened for a limited amount of time. After that it expires and can no longer be accessed. The length of time before it expires depends on what type of Mailock the sender has:
Pro (paid): 365 days from date of send
Free: 21 days from date of send
For more details, see our End User Licence Agreement
The number of days left before the message expires is shown when a message is opened, either on the web or in the Outlook add in, if the message is not set to automatically save a decrypted copy.
If you save a copy of your message, the copy won't expire.
See our article on how to save a copy