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* Add the serialized `Postable` to EML converter code for group mailboxes and associated tests.
* We should converge this with `Messageable` code at some point with a small translator function which converts `Postable` to `Messageable`. Reason being that posts are a subset of messages. Although I feel it's a bit premature right now. Once we have tested this in prod a bit, and we know for a fact that posts are indeed a true subset of messages, we can do this convergence.
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#### Does this PR need a docs update or release note?
- [ ] ✅ Yes, it's included
- [x] 🕐 Yes, but in a later PR
- [ ] ⛔ No
#### Type of change
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- [x] 🌻 Feature
- [ ] 🐛 Bugfix
- [ ] 🗺️ Documentation
- [ ] 🤖 Supportability/Tests
- [ ] 💻 CI/Deployment
- [ ] 🧹 Tech Debt/Cleanup
#### Issue(s)
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* #<issue>
#### Test Plan
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- [ ] 💪 Manual
- [x] ⚡ Unit test
- [ ] 💚 E2E
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