I met this error when I was configing my msmtp on Ubuntu Linux.

msmtp –account bh1scw -d -t < ~/patch/test.patch

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...
ignoring system configuration file /etc/msmtprc: No such file or directory
loaded user configuration file /home/user/.msmtprc
sh: 1: /home/user/.mutt/oauth2.py: Permission denied

All file permission is right and finally I found that we should disable AppArmor

It works after install this package:

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sudo apt install apparmor-utils
sudo aa-disable /etc/apparmor.d/usr.bin.msmtp