Hi @Nuno Silva ,
The ApplicationPoolIdentity accounts are hidden accounts that have dynamically assigned SID's (created and assigned when the ApplicationPool is started). But the accounts live as (hidden) users under the IIS_IUSRS group on the local machine (this makes giving them permissions to the AppPools pretty easy, since you can use the normal GUI interface for perms or use scripts while specifying the local user group).
Use a Metabase Permissions modifier (Metabase Explorer works, so does METAACL.VBS from 2003), Open LM\SMTPSVC and SMTPSVC\1 and add IIS_IUSRS with read permissions to those branches of the metabase.
cscript metaacl.vbs IIS://LOCALHOST/SMTPSVC %computername%\IIS_IUSRS R
cscript metaacl.vbs IIS://LOCALHOST/SMTPSVC/1 %computername%\IIS_IUSRS R
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Best regards,
Bruce Zhang