I tried to configure the mail service postfix according to the official Proxmox VE wiki. After many hours I gave up and started to look for another solution. I then found a post on on getting emails working with ssmtp. It's quite easy to setup and worked very well for me.

First install it with

sudo apt-get install ssmtp

This will install ssmtp and remove postfix. Afterwards we can configure the config file at /etc/ssmtp/ssmtp.conf My config looks similar to this:

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

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

# The full hostname

# 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

I don't have setup an encrypted connection because the server will not send any critical information. When we configure everything as needed we can try to send a testmail with the following command.

echo "Test message from Linux server using ssmtp" | sudo ssmtp -vvv