Install and configure Exim using Fastmail as a smarthost

If you want to receive status updates from your Debian GNU/Linux system in your inbox, you need to employ the help of a mail transfer agent (MTA).

Exim can be configured to use Fastmail as a smarthost and hence ensure the deliverability of your messages. In principle, these instructions should also be applicable to service providers other than Fastmail.

In the following example configuration, debian is the hostname, bookworm the local username and the Fastmail username.

Step 1

Log into your Fastmail account and set up a new app password for SMTP authentication.

Step 2

Install the required packages from the Debian repository.

$ sudo apt-get install exim4-daemon-light

Step 3

Perform the initial configuration using debconf.

$ sudo dpkg-reconfigure exim4-config

Selecting the type of configuration

Select the option for sending mail using a smarthost.

Mail server configuration

General type of mail configuration:

mail sent by smarthost; no local mail


Setting the mail name

Set the system mail name. If you are setting up on a home network, you should use as the domain name.

Mail server configuration

System mail name:


Listening for incoming connections

Only accept connections from the localhost.

Mail Server configuration

IP-addresses to listen on for incoming SMTP connections: ; ::1


Other destinations

Mail Server configuration

Other destinations for which mail is accepted:


Visible domain name

Mail Server configuration

Visible domain name for local users:


Configuring the smarthost

Set the Fastmail SMTP server as the smarthost. Do not miss the double colon!

Mail Server configuration

IP address or host name of the outgoing smarthost:


Minimising DNS-queries

Mail Server configuration

Keep number of DNS-queries minimal (Dial-on-Demand)?


Splitting configuration files

Opt to use a number of small files in /etc/exim4/conf.d/ instead of a single monolithic file in /etc/exim/.

Mail Server configuration

Split config/etc/exim/uration into small files?


Step 4

Make changes to /etc/email-addresses.

$ sudo nano /etc/email-addresses

Provide an outgoing email address for the user bookworm.

# This is /etc/email-addresses. It is part of the exim package
# This file contains email addresses to use for outgoing mail. Any local
# part not in here will be qualified by the system domain as normal.
# It should contain lines of the form:


Step 5

Make changes to /etc/aliases.

$ sudo nano /etc/aliases

Append the following line to send messages for the local user bookworm to the Fastmail address


Step 6

Make changes to /etc/exim4/passwd.client.

$ sudo nano /etc/exim4/passwd.client

Set the Fastmail server as the smarthost. Use the app password you set in Step 1.

# password file used when the local exim is authenticating to a remote
# host as a client.
# see exim4_passwd_client(5) for more documentation
# Example:
### target.mail.server.example:login:password password

Step 7

Generate a local certificate with the following command.

$ sudo bash /usr/share/doc/exim4-base/examples/exim-gencert

Provide your country code as well as server name.

Country Code (2 letters) [US]:your contry code
State or Province Name (full name) []:
Locality Name (eg, city) []:
Organization Name (eg, company; recommended) []:
Organizational Unit Name (eg, section) []:
Server name (eg. ssl.domain.tld; required!!!) []
Email Address []:

Step 8

Modify the default by creating additional configuration files, taking extra care not to miss any punctuation.


$ sudo -- bash -c "echo -e 'MAIN_TLS_ENABLE = yes\nREMOTE_SMTP_SMARTHOST_HOSTS_REQUIRE_TLS = *\nTLS_ON_CONNECT_PORTS = 465\nREQUIRE_PROTOCOL = smtps' >> /etc/exim4/conf.d/main/02_exim4-config_tlsoptions"


$ sudo -- bash -c "echo -e '.ifdef TLS_ON_CONNECT_PORTS\n    tls_on_connect_ports = TLS_ON_CONNECT_PORTS\n.endif' >> /etc/exim4/conf.d/main/04_exim4-config_tlsoptions"


$ sudo -- bash -c "echo -e '.ifdef REQUIRE_PROTOCOL\n protocol = REQUIRE_PROTOCOL\n.endif' >> /etc/exim4/conf.d/transport/31_exim4-config_remote_smtp_smarthost"

Step 9

Update aliases and restart the Exim service.

$ sudo -- bash -c "newaliases&&update-exim4.conf&&systemctl restart exim4"

Step 10

Test your configuration with the following command.

$ sudo echo "Test mail from exim4 on to the local root user and forwarded on to Fastmail" | mail -s "Test Exim" root

Check your Inbox, Linus!