[watcher] Message-ID reused in emails generated with foreach parameter

[immon]

Elasticsearch version (bin/elasticsearch --version): reported in 7.5

Description of the problem including expected versus actual behavior:

When Watcher's email action is run multiple times thanks to foreach parameter, then all generated email messages use the same Message-ID header.

RFC2392 states that Message-ID should be unique:
Both message-id and content-id are required to be globally unique. That is, no two different messages will ever have the same Message-ID addr-spec;

A similar issue was fixed in the past #30112 by adding action_id to Message-ID. With foreach it's not enough since we have one action but generate multiple email messages.

[elasticmachine]

Pinging @elastic/es-core-features (:Core/Features/Watcher)

[jgregmac]

Bummer! I am having this problem as well. It is preventing delivery of Watcher alerts to our ticketing system as the receiving system insists on strict adherence to RFC2392 Message-ID uniqueness.

[kkojouri]

I'm seeing the same issue on our side, getting the same Message-ID even when the body of the messages are different. This results in missing alerts in our ticketing system (which doesn't support Webhook).

[toby-sutor]

This has been addressed here: #56574

[joegallo]

Closing since #56574 has been merged.