MSMTP on Monterey: msmtp: no recipients found

I have MSMTP running flawlessly on four Raspberry Pies. I use it to send email from shell scripts. It works fine as a SMTP client. I installed MSMTP and mailutils on an M1 iMac via MacPorts. I configured MSMTP as follows using the information here: msmtp_gmail_setup /etc/mail.rc:

set append dot save ask crt
ignore Received Message-Id Resent-Message-Id Status Mail-From Return-Path Via
set sendmail=/opt/local/bin/msmtp

The only thing I added was the "set sendmail" line. .msmtprc looks like this:

# Generics
defaults
auth           on
tls            on
# tls_starttls   on
# following is different from ssmtp:
# tls_trust_file /etc/ssl/certs/ca-certificates.crt
# user specific log location, otherwise use /var/log/msmtp.log, however,
# this will create an access violation if you are user pi, and have not changes the access
rights

tls_fingerprint 7D:8B:CD:37

logfile  ~/.msmtp.log

# my  specifics
account        gmail
auth           plain
host           smtp.gmail.com
port           587
from           me@gmail.com
user           me
password       my_password

# Default
account default : gmail

If I run the mail command on the command line it lets me address and compose a message, but I get the following error:

msmtp: no recipients found
Sending data to /opt/local/bin/msmtp failed: Process exited with a non-zero status
cannot send message: Process exited with a non-zero status

There are no entries in ~/.msmtp.log What have I done wrong here?