Email notifications for merge requests not consistently grouped in same conversation thread

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Summary

Email notifications pertaining to the same merge request are not being grouped to the same conversation/thread in Outlook.

Steps to reproduce

Observed on Microsoft Outlook for Mac v16.62. Also observed on the online Microsoft Outlook client (outlook.com).

1. Create a merge request through Gitlab.com.
2. Have other users submit multiple comments on the merge request.
3. Observe in email client that each comment is received as a separate, ungrouped email.

Example Project

What is the current bug behavior?

All email notifications for comments on merge requests show up as separate conversations. Some email notifications, such as for merge request approvals, assignee changes, and reviewer assignments tend to be properly grouped.

What is the expected correct behavior?

All email notifications for merge request updates are grouped by merge request to the same conversation or thread.

Relevant logs and/or screenshots

Output of checks

Results of GitLab environment info

Expand for output related to GitLab environment info

(For installations with omnibus-gitlab package run and paste the output of:
`sudo gitlab-rake gitlab:env:info`)

(For installations from source run and paste the output of:
`sudo -u git -H bundle exec rake gitlab:env:info RAILS_ENV=production`)

Results of GitLab application Check

Expand for output related to the GitLab application check

(For installations with omnibus-gitlab package run and paste the output of:
sudo gitlab-rake gitlab:check SANITIZE=true)
(For installations from source run and paste the output of:
sudo -u git -H bundle exec rake gitlab:check RAILS_ENV=production SANITIZE=true)
(we will only investigate if the tests are passing)

Possible fixes