[PAPP-33723] SMTP: Bugfix - Additional validation on sender email

[leobao-splunk]

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• Bugfix
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Release Notes (REQUIRED)

• No longer allows sender email to have "splunk", "cisco", or "phantom" in the address domain
• Driveby fix to allow email validation to work on single emails

What is the current behavior? (OPTIONAL)

• Currently we only do basic validate on the email to make sure it looks like an email

What is the new behavior? (OPTIONAL)

• Now, for sender emails only, we check to make sure we're not trying to send from an email address that looks like it coming from splunk

Other information (OPTIONAL)

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Screenshots (if relevant)

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