he was poisoned by the envelope glue when he licked it to seal it

@Portia

I once saw a TV detective murder mystery, one of the ones featuring Sherlock Holmes, where a man was killed in a similar situation. He was a rare book collector. A colleague gave him a book he highly prized. The pages were laced with a poison. The man turned the pages by moistening the tip of an index finger with his lips. The habit killed him.

I'm not sure if this meets your criteria but if we substitute the book with an envelope and the finger with lips it could result in the same outcome.

What do you think?

I'm having a Seinfeld flashback 😂
When George's fiancee licked all the wedding invites and....😵

He was murdered. There was poison glue on the envelope he had just licked and sealed.

Anthrax in the envelope.

Heart attack? He could of been poisoned are already had some kind of terminal illness. He could of been shot threw an open window. But to follow up with clue we would have to read or know what was in the envelope,if he received it or was he going to mail it. I hope we can have more than one answer .. if not then oops! I didn't mean to break any rules

He's not really dead.. He's just pining for the Norwegian fjords? 🤔

Carbon monoxide poisoning I told him to buy an alarm 🙄

There was a topical poison on the outside of the envelope.

He saw Randy's face and he died.

He was a friend of the Clintons.

(joke)

He held it up to the light and saw the medical bill his Obama Care didnt cover and had a heart attack ..

Have you been recently watching Jonathan Creek? They had a locked room murder in that using an envelope.

The letter is a response to his lover answering her Dear John - saying he can’t live without her.

I figure poison on the envelope or the bullet hole in the window