mu4e can't send mail "smtpmail-send-it: Sending failed: 502 5.7.0 anonymous login not supported"

Question

I am trying to configure mu4e protonmail.

I can read my messages just fine, but attempting to send a message results in:

smtpmail-send-it: Sending failed: 502 5.7.0 anonymous login not supported

I followed these three links to the letter and they all result in the same error:

https://gist.github.com/azzamsa/174096975df0e16b5bf7e919ba90fce2/
https://doubleloop.net/2019/09/06/emacs-mu4e-mbsync-and-protonmail/
https://gist.github.com/A6GibKm/238b754a4a90051f60906b9efa3e8000

I am running emacs-nox 27.2 on Arch Linux.

If I can provide any additional information, please let me know.

Edit 1: Some more info:

When I attempt to send an email from protonmail (gmail works fine), emacs prompts this message:

Certificate information
Issued by:          127.0.0.1
  Issued to:          Proton Technologies AG
  Hostname:           127.0.0.1
  Public key:         RSA, signature: RSA-SHA256
  Session:            TLS1.3, key: ECDHE-RSA, cipher: AES-256-GCM, mac: AEAD
  Security level:     Medium
  Valid:              From 2021-07-28 to 2041-07-23

The TLS connection to 127.0.0.1:1025 is insecure
for the following reasons:

* certificate signer was not found (self-signed)
* the certificate was signed by an unknown and therefore untrusted authority
* certificate could not be verified

It asks if I want to "Continue Connecting" in light of the above message. When I say yes, that is when I get the anonymous login not supported error that I discussed above.

Edit 2:

For reference, here is my protonmail context:

  (make-mu4e-context
        :name "protonmail"
        :match-func
            (lambda (msg)
            (when msg
                (string-prefix-p "/m-soda-protonmail" (mu4e-message-field msg :maildir))))
        :vars '((user-mail-address . "my email")
                (user-full-name    . "my name")
                (mu4e-drafts-folder  . "/protonmail/Drafts")
                (mu4e-sent-folder  . "/protonmail/Sent")
                (mu4e-refile-folder  . "/protonmail/All Mail")
                (mu4e-trash-folder  . "/protonmail/Trash")

                (message-send-mail-function . smtpmail-send-it)
                (smtpmail-auth-credentials . "/home/marc/.config/mu4e/protonmailpass.gpg")
                (smtpmail-smtp-server . "127.0.0.1")
                (starttls-use-gnutls . t)
                (smtpmail-stream-type . starttls)
                (smtpmail-smtp-service . 1025))))

Answer 1

I found the solution. My problem was probably caused by a bug. Correct me if I'm wrong.

The key referenced by smtpmail-auth-credentials MUST be stored as ~/.authinfo.gpg

Interestingly enough, the value assigned to smtpmail-auth-credentials (mine was "/home/marc/.config/mu4e/protonmailpass.gpg" above) can be anything. The function will always reference ~/.authinfo.gpg regardless of how it is assigned in your config file.

I will do some more digging, and if it is indeed a bug, I will make report to the maintainer of mu4e.

Hope this helps.