Unlicensed usermailbox receiving emails

Question

Hello!

Environment - Hybrid AD with Exchange Online

User was disabled almost 1 year ago and the M365 E3 license was removed. The problem is that this mailbox is still able to receive e-mails (not the only affected mailbox). From what I know, mailbox should be held active for 30 days after the license is removed and then it should be deleted/moved to inactive.

Now, since the company is required by law to keep the e-mails, there is a retention policy applied to prevent them from being deleted. There is a bunch of inactive mailboxes and I was thinking that the mailbox will be converted to this state after license is removed. From what I have found, GUID of the retention policy is the same as the one that causes problem in the admin center message below, so it looks like that this retention policy, that should keep the e-mails, is also keeping mailbox active.

My questions:

1. What is the best practice to prevent mailbox to receive e-mails in this case?
2. Why does retention policy keep the mailbox active?

The end goal is to keep e-mails from the users and disable mailbox from receiving new e-mails. I know that deleting/unsyncing the user object from AD will transfer mailbox to Inactive mailboxes, but I've always only disabled users in AD when they left the company.

Admin center message:
Exchange: The execution of cmdlet Disable-Mailbox failed. Exchange can't disable the mailbox "username" because it is on In-Place Hold. ComplianceTagHoldApplied: false, DelayHoldApplied: false, DelayReleaseHoldApplied: false, OrganizationPolicies Applied: mbx423c16207a5f441ba43e8e5ad645f532:2, UserPolicies Applied: , restrictivePolicies Applied: . For more information, please see the PowerShell cmdlets, Get-OrganizationConfig <>.;

Get-mailbox result:
LitigationHoldEnabled : False
RetentionHoldEnabled : False
EndDateForRetentionHold :
StartDateForRetentionHold :
LitigationHoldDate :
LitigationHoldOwner :
ComplianceTagHoldApplied : False
DelayHoldApplied : False
DelayReleaseHoldApplied : False
LitigationHoldDuration : Unlimited
SCLDeleteThreshold :
SCLRejectThreshold :
SCLQuarantineThreshold :
SCLJunkThreshold :
InPlaceHolds : {}
RecipientThrottlingThreshold : Standard

Kind regards,
Domagoj