Log Message:
Update msmtp to 1.4.21.
Based on PR#43566, with additional clean up, exactly enable/disable options.

Version 1.4.21:
- No significant changes.

Version 1.4.20:
- Added support for authentication mechanism SCRAM-SHA-1 via GNU SASL.
- The new command tls_fingerprint allows one to trust one particular TLS
  certificate, in case tls_trust_file cannot be used for some reason.
- The new script msmtp-gnome-tool.py manages Gnome Keyring passwords for msmtp.

Version 1.4.19:
- When using OpenSSL, msmtp now correctly handles NUL characters in the Common
  Name and Subject Alternative Name fields of certificates. This fixes a
  security problem. Note that msmtp is not affected by this problem if GnuTLS is
  used.