PAPP-34625: Bugfix - Remove email domain validation

[leobao-splunk]

Please ensure your pull request (PR) adheres to the following guidelines:

• Please refer to our contributing documentation for any questions on submitting a pull request, link: Contribution Guide

Pull Request Checklist

Please check if your PR fulfills the following requirements:

• Testing of all the changes has been performed (for bug fixes / features)
• The manual_readme_content.md has been reviewed and added / updated if needed (for bug fixes / features)
• Use the following format for the PR description: <App Name>: <PR Type> - <PR Description>
• Provide release notes as part of the PR submission which describe high level points about the changes for the upcoming GA release.
• Verify all checks are passing.
• Do NOT use the next branch of the forked repo. Create separate feature branch for raising the PR.
• Do NOT submit updates to dependencies unless it fixes an issue.

Pull Request Type

Please check the type of change your PR introduces:

• New App
• Bugfix
• Feature
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no api changes)
• Documentation
• Other (please describe):

Security Considerations (REQUIRED)

• If you are exposing any endpoints using a REST handler,
  please document them in the manual_readme_content.md.
• If this is a new connector or you are adding new actions
  • Please document in the manual_readme_content.md all methods (eg, OAuth) used to authenticate
    with the service that the connector is integrating with.
  • If any actions are unable to run on SOAR Cloud, please document this in the manual_readme_content.md.
• Are you introducing any new cryptography modules? If yes, please elaborate their purpose:
• Are you are accessing the file system? If yes, please verify that you are only accessing paths returned through
  the Vault API.
• Are you are marking code to be ignored by Semgrep with nosemgrep?
  If yes, please provide justification in an additional comment next to the ignored code.

Release Notes (REQUIRED)

Remove the email domain validation as it's too restrictive on what's allowed.

What is the current behavior? (OPTIONAL)

Will block all sender emails with 'phantom', 'cisco', or 'splunk' in it's domain.

What is the new behavior? (OPTIONAL)

No longer imposes the above restriction

Other information (OPTIONAL)

• Any other information that is important to this PR such as screenshots of how the component looks before and after the change.

Pay close attention to (OPTIONAL)

• Any specific code change or test case points which must be addressed/reviewed at the time of GA release.

Screenshots (if relevant)

---

Thanks for contributing!

[alexa-phantom]

Please add release notes like https://github.com/splunk-soar-connectors/smtp/blob/next/release_notes/3.2.0.md?plain=1