Hi @Glenn Maxwell
According to my research, in Microsoft Teams, the application of retention policy depends on where the message is stored and how the policy is configured. For guest accounts in B2B collaboration, the messages are stored in the mailbox of the tenant to which the guest account belongs.
Therefore, if the tenant retention policy of the guest account is 1 month, then these messages will be deleted from the mailbox of the guest account after 1 month. However, this does not affect the retention policy in your tenant. Users in your tenant can still see these messages in their Teams app until your 6-month retention period ends.
Ref: Learn about retention for Microsoft Teams
If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.