This PR was merged into the 4.4 branch.

Discussion
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[Mailer] Rename an exception class

| Q             | A
| ------------- | ---
| Branch?       | 4.4
| Bug fix?      | yes
| New feature?  | no <!-- please update src/**/CHANGELOG.md files -->
| Deprecations? | no <!-- please update UPGRADE-*.md and src/**/CHANGELOG.md files -->
| Tickets       | n/a
| License       | MIT
| Doc PR        | n/a
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Commits
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2c58dcc [Mailer] Rename an exception class