Mixin Messenger uses Signal protocol, it means that the messages are encrypted and once the messages are delivered, the servers will delete the message records, then the messages saved on the ends. Then what is going on when the receivers are not online?
There're 2 kinds of messages on Mixin Messenger: private messages are encrypted and normal messages sent by bots. The private messages include personal messages and group (256 members) messages. The servers will keep all private messages until the receivers are online to decrypt the messages, then the messages will be deleted from the servers.
The messages sent by bots will be kept 7 days. If you don't open Mixin Messenger for a long time, you will lose messages.