Setup Sendmail with Sendgrid

How to setup Sendmail MTA (Mail Transport Agent) on server to use Sendgrid SMTP.

Prerequisites

Install required packages.

wo stack install --sendmail
apt install libsasl2-modules

1. Sendgrid API key

Create a new Sendgrid API key (if not already created).

2. Create Auth File

mkdir /etc/mail/authinfo
chmod 700 /etc/mail/authinfo

Create /etc/mail/authinfo/smtp-auth and add the following content:

AuthInfo: "U:root" "I:apikey" "P:API_KEY"

Create a hash map file of above created auth file.

makemap hash /etc/mail/authinfo/smtp-auth < /etc/mail/authinfo/smtp-auth

3. Configure Sendmail with SMART_HOST

Add the following configuration lines into /etc/mail/sendmail.mc after MAILER_DEFINITIONS and before MAILER(`local')dnl at the bottom.

define(`SMART_HOST',`[smtp.sendgrid.com]')dnl
define(`RELAY_MAILER_ARGS', `TCP $h 587')dnl
define(`ESMTP_MAILER_ARGS', `TCP $h 587')dnl
define(`confAUTH_OPTIONS', `A p')dnl
TRUST_AUTH_MECH(`EXTERNAL DIGEST-MD5 CRAM-MD5 LOGIN PLAIN')dnl
define(`confAUTH_MECHANISMS', `EXTERNAL GSSAPI DIGEST-MD5 CRAM-MD5 LOGIN PLAIN')dnl
FEATURE(`authinfo',`hash -o /etc/mail/authinfo/smtp-auth.db')dnl

Re-build sendmail's configuration:

make -C /etc/mail

4. Verify setup

service sendmail reload

Send test email to name@domain.tld:

sendmail name@domain.tld
Subject: Test
This is the body of the test email.

Press control + d to send.