https://wiki.contribs.org/Email#I_can.27t_receive.2Fsend_email_from_my_application_.28ACT.21.2C_vTiger.2C_MS_Outlook.2C_etc.29

This SMTP error 552 may keep you from connecting Outlook 2016 to an SME Server IMAP server.

To disable the check for the 'Date header' on the internal interface:

mkdir -p /etc/e-smith/templates-custom/var/service/qpsmtpd/config/peers/local
cd /etc/e-smith/templates-custom/var/service/qpsmtpd/config/peers/local
echo "# 17check_basicheaders disabled by custom template" > 17check_basicheaders
signal-event email-update