3392043 - Email notification not triggered, but Fiori launchpad notification works

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1.
Please note this KBA is only relevant for E-Mail notification of fiori push notification. 
This KBA is not suitable for Workflow E-Mail Notification feature, which is triggered by SAP_WORKFLOW_OUTPUT_MANAGEMENT.
Both features (fiori E-Mail push notification and Workflow E-Mail Notification)  have nothing to do with each other.

2.
When you are using workflow to create fiori push notification, there is no E-Mail supported for this scenario.
Workflow as provider hasn't yet implemented it like:
https://blogs.sap.com/2020/12/01/enable-email-notification-in-notification-channel-framework/
So for workflow notification in fiori, there is no such setting:
Launchpad -> User -> Settings -> Notifications, Email notification

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Email, Mail, E-mail notification not triggered, but Fiori launchpad notification works. 

Symptom 1:

Table /IWNGW/NRECIP entry's "DO NOT DELIVER EMAIL" = X, example:

MANDT                   XXX
NOTIF ID                FA163E31DE921EEE9CAD2BA50B9CBC19
USER ID                 XXXXXX

IS READ
IS SEEN
PRIO DEFAULT            40-HIGH
DO NOT DELIVER          
DO NOT DELIVER MOB
DO NOT DELIVER EMAIL    X

Symptom 2:

/IWNGW/NTYPE_TT mail entry is not maintained, example:

MANDT               XXX
TYPE ID             XXX
SPRAS               EN

TEMPL PUBLIC        A leave request requires your attention
TEMPL SENSITIVE     Leave request {lr_number} by {requester
TEMPL GROUPED       You have {_group_count} leave requests
DESCRIPTION         Leave Request
SUBTITLE
TEMPL EMAIL SUB             ==>empty
TEMPL EMAIL TEXT            ==>empty
TEMPL EMAIL HTML           ==>empty

/IWNGW/NRECIP."DO NOT DELIVER EMAIL" is also not checked  

Notification

/IWNGW/BEP_DEMO, /IWNGW/NRECIP, DO NOT DELIVER EMAIL, DO NOT DELIVER, DO NOT DELIVER MOB, e-mail, mail, email, /IWNGW/NTYPE_TT, "TEMPL EMAIL SUB", "TEMPL EMAIL TEXT", "TEMPL EMAIL HTML", /IWNGW/CL_OUT_NOTIF_EMAIL, /IWNGW/IF_HUB_OUT_PUSH_CHANNEL, SAP_WORKFLOW_OUTPUT_MANAGEMENT , KBA , OPU-GW-NOT , Notification , Problem