AAD Issue with Distribution groups

Question

Hi,

Hopefully someone can help. The below is all lab so any suggestions, I can do with no worry of breaking anything!

I currently have AAD Connect configured for 2 domains. DomainA hosts AAD Connect and DomainB is added in as an additional domain. All working fine from a sync perspective and I have some rules in place that translate DomainA objects to use the DomainB domain etc.

An issue I am having is, I want distribution groups synced from DomainB to utilise a domain that is hosted in DomainA's AAD tenant. I created a rule for users that would map DomainBs UPN to synced DomainA users with no issue.

The rule I have for distis is as below (exported from AAD Connect and Ive highlighted some suspect rows that may be causing an issue):

New-ADSyncRule -Name 'Modify Distribution Group email'
-Identifier '83714098-ac67-40ec-8671-2e20335e5c04' -Description ''
-Direction 'Inbound' -Precedence 42
-PrecedenceAfter '00000000-0000-0000-0000-000000000000' -PrecedenceBefore '00000000-0000-0000-0000-000000000000'
-SourceObjectType 'group' -TargetObjectType 'group'
-Connector '58f89d2b-e399-46fe-b8b3-be7202596888' -LinkType 'Join'
-SoftDeleteExpiryInterval 0 -ImmutableTag ''
-OutVariable syncRule

Add-ADSyncAttributeFlowMapping -SynchronizationRule $syncRule[0]
-Destination 'mail' -FlowType 'Expression'
-ValueMergeType 'Update' **-Expression 'Word([mail],1,"@") & "@DomainA.com"'**
-OutVariable syncRule

New-Object -TypeName 'Microsoft.IdentityManagement.PowerShell.ObjectModel.ScopeCondition'
-ArgumentList 'dn','DC=Domain A AD','ENDSWITH' `
-OutVariable condition0

Add-ADSyncScopeConditionGroup -SynchronizationRule $syncRule[0]
-ScopeConditions @($condition0[0]) `
-OutVariable syncRule

Add-ADSyncRule `
-SynchronizationRule $syncRule[0]

Get-ADSyncRule `
-Identifier '83714098-ac67-40ec-8671-2e20335e5c04'