Currently the SDM email delivery is done using the RFC1891 rules.
This can be verified by running pdm_mail -h on any SDM install - including r14.1:
With that, field message_id field is empty in the message header.
However, some email servers (POP/IMAP) in the internet are requiring this field exists in the messages generated by the email clients.
This was optional, but currently all email servers are adopting this, which is specified in RFC2822.
What happens when this field is empty?
In short, the message_id is verified by the email servers to validate from where the message was delivered and if it has already been delivered by the email server for the same user.
With this field empty, some SDM notifications are not delivered to the recipients if two (or more) SDM notifications are triggered within the same second.
The mail server understands all are the same message and blocks the additional notifications (which are, in fact, different).
In such situation, if the message_id was filled, the notifications would have reached the recipients.
From the SDM perspective, this may cause Analysts to get tickets with SLA expired for not being notified.
This idea requests SDM email delivery be changed to accommodate RFC2822 rules.