Email relaying – smarthost setup

Overview of alternatives for relaying emails

Free SMTP Servers for Sending Emails – 2021
Free SMTP Servers & Free SMTP Relay Services

Relaying emails using a Gmail account

https://support.google.com/a/answer/2956491?hl=en

Main disadvantages:

• Low daily (24hr rolling) limit of 500 messages
• Sends as (and takes replies on) the gmail account used for relaying

All limitations: https://support.google.com/a/answer/166852?hl=en

Setup guide for UNIX-like OSes (msmtp / msmtp-mta)

msmtp/msmtp-mta is a lightweight solution for relaying mail through gmail or any other external service.
I have successfully followed this guide:
Using MSMTP with Google SMTP Relay on Ubuntu 20.04 (DFT blog)
Another guide:
Use external SMTP server for system mails on Linux
See also:
msmtp documentation (Arch Linux Wiki)

Setup guide for multiple UNIX-ish OSes (for Postfix)

https://www.howtoforge.com/tutorial/configure-postfix-to-use-gmail-as-a-mail-relay/

Quick setup (Ubuntu and other):

Gmail account: allow insecure apps
https://myaccount.google.com/security?gar=1

Required software: postfix and mailutils

/etc/postfix/sasl_passwd:

[smtp.gmail.com]:587    username@gmail.com:password

Protect and process password file:

chmod 600 /etc/postfix/sasl_passwd
postmap /etc/postfix/sasl_passwd

Configure postfix:
/etc/postfix/main.cf:

relayhost = [smtp.gmail.com]:587
smtp_use_tls = yes
smtp_sasl_auth_enable = yes
smtp_sasl_security_options =
smtp_sasl_password_maps = hash:/etc/postfix/sasl_passwd
smtp_tls_CAfile = /etc/ssl/certs/ca-certificates.crt

Restart Postfix

systemctl restart postfix.service

or

postfix stop
postfix start

WP plugin for setting sender name (address can not be changed): CB Mail Sender

Relaying emails using Elastic-Email

Elastic-Email is primarily a email marketing system like MailChimp, but can also act as a SMTP relay.

Main advantages:

• Domains and email address can be confgured as approved senders, email will be sent using one of the confirmed senders
• API available

Main disadvantages

• Impossible to get rid of the “unsubscribe” link in every email sent
• Tracking portions are added to links, and links are destroyed (no longer clickable) in the sent emails

Setup guide

https://help.elasticemail.com/en/articles/2388043-how-to-configure-postfix-with-elastic-email

WP plugin: Elastic Email Sender

Relaying through Oracle Cloud

https://docs.cloud.oracle.com/en-us/iaas/Content/Email/Concepts/overview.htm

Relaying mail from WordPress

The system-wide configurations and alternatives above work well for WordPress too, but you might want to finetune some settings for the emails being sent from WP. That includes the sender name, reply-to email, or you might even want to send these with another email account (or email relay service).
I found the plugin “Post SMTP Mailer/Email Log” (Post SMTP / PostmanSMTP) be the best suitable for this (there are many others available, so this might NOT be the best for everyone):