Exactly what it sounds like.

It's not a super common term, so it really doesn't have a firm definition. But it's an email that you either get or receive that does damage to your professional reputation such that your career is effectively over, either in general or at a particular place.

For example, sending an email to the entire company criticizing upper management's decisions is likely to end your career at a particular place.

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That example is way too tame. Criticizing upper management's decisions might get you a reprimand and might make it hard for you to get a promotion but it's unlikely to be career-ending.

Career-ending would be:

• Overt racism or sexism (e.g. I don't like working with X because they're a <racial slur>)
• Taking credit for someone else's work
• Embezzling money
• Committing fraud
• Making up outright lies to explain a negative outcome (e.g. we failed to sell more widgets because our whole sales team quit halfway through the quarter)
• Offering or accepting a bribe

Sharing porn with a co-worker and “accidentally” sending it to the “All Employees” distribution list.

There's two different ways you can use it, internal and external. Internal is you sent an email that was so repulsive to the business that there is nothing that you ever earn another promotion (or more likely keep your job). They're career ending in the sense that you'll never work for that company again and thus your prior career there is over.

Or if that email gets enough public attention that it becomes public knowledge, or at least knowledge of it persists on the internet, and the content is so bad that you'll never work in your given field again. Mark Bankston, Alex Jones's lawyer, mishandled a discovery request. It was supposed to be a limited amount of data from Jones's cell phone and instead sent over an image of the phone entirely and failed to challenge the release properly. That let the prosecution prove that Jones had repeatedly lied during the trial and lead to a billion dollar fine. That lawyer will never spend another day in court, regardless if he keeps his license or not.

It means somebody put something so unprofessional in an email that it ended their career. Like, most places that takes a MAJOR gaffe.

We're talking hitting reply-all on a dept wide email with some NSFW content or something. Or accidentally including proof you've been scamming the company.

We have in our business what I call "Resume Generating Events" or an RGE.

Example: You hear that James has not done backups for over a year and accounting just lost the Sql Database. RGE for James.

This is what you have done is so bad, whatever it may be, you need to go ahead and start printing your resume out.