I looked for the Privacy Settings for this user in ACP whether he unchecked the box "Receive email when a new conversation message is received" within his "Contact Details". But this box seems to be missing in ACP.
why do we see in ACP if the user accepts receiving the Newsletter (site mailings), but we do not see whether the user accepts to receive "E-mail notifications" on Private Conversations.
Looks like this option has been missed to be included in ACP.
that should be put into "User Privacy Options" within ACP for a specific user.
reference is this thread