defaults
auth           on
tls            on
tls_trust_file /etc/ssl/certs/ca-certificates.crt
logfile        /etc/msmtp/msmtp.log
aliases        /etc/aliases

account        sendgrid
host           smtp.sendgrid.net
from           noreply@your.domain
port           587
user           apikey
password       your.api.secret

account default : sendgrid

Note that I made a symlink of /etc/msmtprc to /etc/msmtp/msmtprc, so it's easier to mount the whole folder into docker.