IssueTracker

Please provide a short description of the issue:

Having a mime message with the following structure

multipart/alternative
|_ content-type: text/plain
|_ content-type: text/html

with customized content per alternative, sending the email with Gmail REST API changes the text/plain part of the email that the recipient receives but the correct (not changed email) is stored in sender's sent folder.

Seems that the text/plain alternative is automatically generated from the text/html part of the email.

What steps will reproduce the problem?
1. Create an multipart/alternative email with custom text plain and text html parts.
2. Send the email with Gmail REST API from user A to B.
3. Check B inbox email using "Show Original" or "Message text garbled?".

What is the expected output? What do you see instead?

User B should receive the exact email that user A sends, however the current result is that B receives a different email, "showing the original" or "Message text garbled?" shows a different text/plain than the sent email.


Please provide any additional information below.

Although the problem is different maybe is related to issue 36758842, because if you send the exact email using SMTP the email received correctly, that is, with the text/plain part untouched.

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