Fix proofpoint detection case-insensitivity

[amori1729]

Summary

• fix Proofpoint MX detection not to be case sensitive
• add regression test for Proofpoint detection

Testing

• cargo test --quiet

---

https://chatgpt.com/codex/tasks/task_e_683f5769c19c8325878fd187364759c0

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}