I have a Raspberry Pi that I am using for some Home Automation Application. It needs to send email to me when various events happen using the HA App User Name, plus some general system status information from root.

I created an email account relay@mudomain.systems and have the RPi use that.

In /etc/main.cf I have
mydomain = mydomain.systems
relayhost = [smtp.dynu.com]:587

In /etc/relay_password I have
[smtp.dynu.com]:587 retay@mydomain.systems:password

Part 2: I thought there was limit to the size of a post, but it turns out it does not like a certain sequence of characters and kept tell me to enter a valid message. So here is the rest of what I tried to post.

When root sends email the mail.log file shows the from=(root@rpi.mydomain.systems). It appears the connection is made and the authentication is done and then then smtp.dynu.com returns a status of 554 Invalid From Address.

So is it complaining that root@rpi.mydomaon.systems does not match relay@mydomain.systems?

Or it cannot validate rpi.mydoman.systems?