SSMTP returns error 550

Question

I have a Raspberry Pi with Raspian installed on it and I am experiencing troubles sending out emails.
I have installed SSMTP and usually inside my scripts I'm sending out emails to notify me when a job is completed:

echo "$(date) Job completed" | mail -s "My subject" [email protected]

Everything was working fine until a couple of months ago, when I stopped receiving emails. I tried to manually launch the script and I receive this error:

send-mail: 550 Your authenticating ID must match your sending address.

I suspect there's something wrong in my SSMTP configuration, that at the beginning my email provider wasn't checking and now it's not working. Here you can find my ssmtp.conf file:

#
# Config file for sSMTP sendmail
#
# The person who gets all mail for userids < 1000
# Make this empty to disable rewriting.
root=postmaster

# The place where the mail goes. The actual machine name is required no
# MX records are consulted. Commonly mailhosts are named mail.domain.com
mailhub=mail

# Where will the mail seem to come from?
#rewriteDomain=

# The full hostname
hostname=raspberrypi

# Are users allowed to set their own From: address?
# YES - Allow the user to specify their own From: address
# NO - Use the system generated From: address
FromLineOverride=YES
AuthUser=myUser
AuthPass=myPassword
mailhub=mysite.smtp.com:587
UseSTARTTLS=YES

I think there's something wrong, but I'm not a sysadmin and I don't have so much experience with mail services, any suggestions?

Answer 1

authenticating ID must match your sending address.

The server mysite.smtp.com: (or more specifically, the MTA its running) rejects your mail, because you authenticate as one user (myUser), but send a mail as another user.