[Bug 1853007]

[Rob Lemley]

I can confirm this as well. 
This appears to be in the message sending code. I would not be surprised if saving drafts also triggers these files as saving a draft shares a lot ofg the same code as actually sending.

Today at 14:31 local time, I sent the email about building Thunderbird
85.0b3. And I have several files in tmp from that time that contain that
message in various states:

```
-rw------- 1 rob  rob     3959 Dec 30 14:31 key-3.asc
-rw------- 1 rob  rob    78973 Dec 30 14:31 nscopy-2.tmp
-rw------- 1 rob  rob     3753 Dec 30 14:31 nsemail-2.html
-rw------- 1 rob  rob    78973 Dec 30 14:31 nsemail-3.eml
-rw------- 1 rob  rob    46469 Dec 30 14:31 nsmail-2.tmp
-rw------- 1 rob  rob      517 Dec 30 14:31 nsmail-3.tmp
```

* **key-3.asc** - This is a copy of my public key
* **nscopy-2.tmp** - The entire email, headers and all attachments as one would expect to find in a maildir/mbox file
* **nsemail-2.html** - The email body in HTML format
* **nsemail-3.eml** - The entire email, same as nscopy-2.tmp
* **nsmail-2.tmp** - The release notes that were included in the email (and html attachment, so this file is HTML)
** *nsmail-3.tmp** - The email body in plain text

Based on the files left behind, it looks like when a message is sent,
the various mime parts are written out to temporary files to assemble
the final message before sending. That produces nsemail-3.eml.
nscopy-2.tmp is maybe because of the copy to the sent folder.

This is with Thunderbird 78.6.0.

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Title:
  nsemail and nscopy files left in /tmp

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