Linux Configure Sendmail for Email Service

Preface

Due to development needs, I built a wiki系统 (wiki system) and used email subscription, so I researched various methods for sending emails.

Installation

There's nothing much to say, just install the necessary components first.

apt install sendmail-bin
apt install sendmail
apt install sendmail-cf # Configuration tool

Configure sendmail

sendmail is by default set up to send emails only locally on the machine. Therefore, it needs to be modified to allow sending emails across the entire Internet: Modify the sendmail configuration macro file, located at /etc/mail/sendmail.mc

Find

DAEMON_OPTIONS('Family=inet,  Name=MTA-v4, Port=smtp, Addr=127.0.0.1')dnl

Change Addr=127.0.0.1 to Addr=0.0.0.0, which allows connections to any server.

Save the modified file, then back up the configuration file:

cd /etc/mail
mv sendmail.cf sendmail.cf~

Next, generate a new configuration file:

m4 sendmail.mc > sendmail.cf

Restart sendmail for the changes to take effect.

Configuring Domain Names

Configuring sendmail to use a legitimate domain name involves two steps: modifying sendmail's configuration file and adding the domain's MX record.

sendmail can directly use the -r account@domain.com parameter to send emails from any source address. However, most mainstream email providers will compare the source address with the reverse DNS resolution of the IP. If the resolution fails or the resolved IP doesn't match, the email might be classified as SPAM or, in worse cases, be outright rejected.

The MX Record (Mail Exchanger Record) is primarily used for receiving emails. When delivering a new email, the system queries the MX record of the recipient's domain and uses the IP address obtained from the MX record to proceed with delivery. Additionally, email providers often compare the source address with the MX record when receiving emails, using it as one of the criteria for identifying spam.

Step 1: Add the Domain

(1) Add the domain to the local-host-names file

Open: /etc/mail/local-host-names Add:

deali.cn

(2) Modify the submit.cf file

Open: vi /etc/mail/submit.cf, find the line #Dj$w.Foo.COM, and modify it to

Djdeali.cn

Attention! It must be in the format of Dj + Domain!

At this point, the sendmail email naming configuration is complete. Restart sendmail to apply the configuration.

Step 2: Add Domain MX Records

Navigate to the page where you can modify your domain information (this page varies by domain registrar). The changes may take up to 24 hours to take effect, depending on the registrar.

(1) Add an A record for the mail subdomain, pointing directly to your mail server's static IP address:

(2) Add (or modify) your domain's MX record in the following format:

(3) Use nslookup to verify that the MX record correctly resolves to your mail server.

nslookup
> set q=mx
> deali.cn
Server:         8.8.8.8
Address:        8.8.8.8#53

Non-authoritative answer:
deali.cn        mail exchanger = 5 deali.cn.

Authoritative answers can be found from:

Email Send Test

Enter the following command: sendmail -t <<EOF

A > prompt will appear, then input the content in the format below (press Enter after each line):

From:Mail test <test@yourdomain.com>
To:xxxx@qq.com
Subject:Test
test
EOF

See the question

Relaying denied（Relaying denied）

When the error 550 5.7.1 <xxx@163.com>... Relaying denied. IP name lookup failed [192.168.1.133] occurs, the reason is that sendmail is being used as a mail relay. To resolve this, you need to add the client's IP address to the access configuration file.

Open: /etc/mail/access, and add the following:

Connect:Client_IP                   RELAY

Then, regenerate the access permissions database:

cd /etc/mail/
makemap hash access.db < access 

Sent Emails Being Identified as Spam by Other MTAs

Set up domain pointing (link to the previous section) and add SPF records Operation Tutorial Also, pay attention to controlling the email sending frequency.

Where to Check Sendmail Logs?

Log location: /var/log/maillog

To adjust the log level (verbosity): modify the configuration file define(confLOG_LEVEL',16')dnl(default is9), or specify when calling the command: sendmail -O LogLevel=14`

Log Level Explanation

What do sendmail logs mean?

Note that sendmail does not guarantee that the sent email will always be delivered to the recipient, so the information in the logs is only the connection information fed back by the receiving end's MTA. For the specific format, feel free to read through it—–sendmail log format

Sendmail Performance Parameters

http://www.5dmail.net/html/2008-4-27/200842733006.htm

Some related websites and articles

http://www.5dmail.net/ Sendmail official site All about email sending Viewing email headers