Get current Settings
Get-Mailbox service@contoso.de | Format-List SingleItemRecoveryEnabled,RetainDeletedItemsFor
or use
Get-mailbox service@contoso.de | fl *retention*,*retain*
Set New Settings
Set-Mailbox -Identity service@contoso.de -SingleItemRecoveryEnabled $false -RetainDeletedItemsFor 1
#Purges folder deletes after 1 day
Delete Items in Dumpster
Search-mailbox -identity service@contoso.de -SearchDumpsterOnly -DeleteContent
View Recoverable items in Dumpster
Get-MailboxFolderStatistics service@contoso.de -FolderScope RecoverableItems | Format-List Name,FolderAndSubfolderSize