Mailsac will delete messages on regular intervals. One way around this is to create an account and star messages so they will be saved.
Use the star symbol ★
When you’re logged in, you will see the star symbol next to each message. Just click it to toggle saving.
When you star a message, it counts towards your overall storage. But once it’s starred, you can be sure it will not get recycled, so you can refer back to it later.
Your user dashboard has links to all inboxes where you starred messages.
You can star messages on any address, including those you own or other public addresses.