How to Install Sendmail on Ubuntu 18.04 & 16.04 LTS

Sendmail is a general purpose email routing facility used for email transport over the Internet. It includes SMTP (Simple Mail Transfer Protocol) for the mail-transfer and email delivery. Most of the system administrators preferred to use Sendmail server as MTA than other MTAs. You can also use Sendmail server to send the email via external SMTP servers like Gmail, Amazon SES, MailChimp etc.

1. Remove Postfix

Postfix is the default SMTP service pre-installed on Ubuntu operating systems. And you are willing to use Sendmail server on your system.

First of all, remove the existing postfix installation on Ubuntu.

sudo systemctl stop postfix
sudo apt remove postfix && apt purge postfix

2. Install Sendmail

If you don’t have installed Sendmail using the following command to install Sendmail with other required packages using yum package manager.

sudo apt install sendmail

3. Configure Sendmail Server

The execute the sendmailconfig command to complete the basic configuration.

sudo sendmailconfig

Select all options to ‘Y’ and press enter. Wait for the command finish.

Your server is ready for sending emails. You can use the Linux command line or PHP script to send emails.

4. Receive Incomming Emails

Edit /etc/mail/sendmail.mc file and comment below line to allow receiving an email from anywhere. To comment a line in sendmail.mc, just put dnl keyword at the start of the line.

dnl DAEMON_OPTIONS(`Family=inet,  Name=MTA-v4, Port=smtp, Addr=127.0.0.1')dnl
dnl DAEMON_OPTIONS(`Family=inet,  Name=MSP-v4, Port=submission, M=Ea, Addr=127.0.0.1')dnl

Then add your domain names to /etc/mail/local-host-names file.

cat /etc/mail/local-host-names

tecadmin.net
mail.tecadmin.net
localhost
localhost.localdomain

Now use m4 is a macro processor to compile the Sendmail configuration files. m4 is stream-based, that is, it doesn’t understand about lines.

sudo m4 /etc/mail/sendmail.mc > /etc/mail/sendmail.cf

Restart Sendmail service

sudo systemctl restart sendmail

Your system is ready for incoming emails.

5. Configure Domain-based E-mail Routing

As we read above that virtusertable file used for aliasing, allowing multiple virtual domains to be hosted on one machine.

1. All emails addressed to @example.com domain delivered to support@mydomain.com
```
@example.com support@mydomain.com
```
2. All emails addressed to support@mydomain.com will forward to local user jack.
```
support@mydomain.com  jack
```
3. All emails addressed to @mydomain.com will forward to domain @otherdomain.com with corresponding usernames.
```
@mydomain.com    %1@otherdomain.com
```
4. All emails addressed to @otherdomain.com will be rejected my mail server with acknowledging sender with the message
```
@otherdomain.com 	 error:nouser User unknown
```

After making all changes in virtusertable execute following command to create updated virtusertable.db file containing the new configuration.

sudo makemap hash /etc/mail/virtusertable

Now restart Sendmail service