How to set up a SMTP relay on your Proxmox node

5-minute craft, specially useful for those self-hosted Proxmox users who seek sending e-mails to their remote inboxes directly from their nodes:

Assumptions

Your Proxmox run behind a NAT / is not directly accessible on the Internet so you find trouble sending native system e-mails by default
You have an already existing e-mail address you would want to make your Proxmox host use to send mails to you
The connection method to the “sender” e-mail address will be using password authentication

Step by step guide

First off, install some dependencies. Run as root:
```
apt-get update
apt-get install libsasl2-modules
```

Edit the /etc/postfix/main.cf file like this (replace relayhost’s key value to suit your needs):

# See /usr/share/postfix/main.cf.dist for a commented, more complete version

myhostname=pve.local

smtpd_banner = $myhostname ESMTP $mail_name (Debian/GNU)
biff = no

# appending .domain is the MUA's job.
append_dot_mydomain = no

# Uncomment the next line to generate "delayed mail" warnings
#delay_warning_time = 4h

alias_maps = hash:/etc/aliases
alias_database = hash:/etc/aliases
#mydestination = $myhostname, localhost.$mydomain, localhost
mynetworks = 127.0.0.0/8
inet_interfaces = loopback-only
recipient_delimiter = +

sender_canonical_classes = envelope_sender, header_sender
sender_canonical_maps =  regexp:/etc/postfix/sender_canonical_maps
smtp_header_checks = regexp:/etc/postfix/header_check

relayhost = <smtp.mailserver.com>:<smtp_server_port>
smtp_use_tls = yes
smtp_sasl_auth_enable = yes
smtp_sasl_security_options = noanonymous
smtp_tls_wrappermode = yes
smtp_tls_security_level = encrypt
smtp_sasl_password_maps = hash:/etc/postfix/sasl_passwd
smtp_tls_CAfile = /etc/ssl/certs/ca-certificates.crt

Edit or create the /etc/postfix/sasl_passwd file, that will contain the following line (remember to place your own values there):
```
<smtp.mailserver.com>:<smtp_server_port>    <sender>@<mailserver.com>:<password>
```
Change file permissions with:
```
chmod 600 /etc/postfix/sasl_passwd
```
Run:
```
postmap /etc/postfix/sasl_passwd
```
Edit or create the file /etc/postfix/sender_canonical_maps containing:
```
/.+/    <sender>@<mailserver.com>
```
Edit or create the file /etc/postfix/header_check with the following:
```
/From:.*/ REPLACE From: <sender>@<mailserver.com>
```
Apply changes by restarting Postfix service:
```
systemctl restart postfix.service
```

And that’s it. You can always manually test if it works by running:

echo "This is a test e-mail" | mail -s "Testing" <receiver>@<mailserver.com>

See you next time!