The author voluntarily contributed this tutorial as a part of Pepipost Write to Contribute program.

Introduction

Using the Gmail\'s SMTP Server, you can send emails to any domain using your Gmail Credentials. Following are some email sending limit criteria: + Google limits the number of recipients in a single email and the number of emails that can be sent per day. + The current limit is 500 Emails in a day or 500 recipients in a single email. + On reaching threshold limits, you won\'t be able to send messages for the next 24 hours. + After the suspension period, the counter gets reset automatically, and the user can resume sending Emails. + For more information about email sending limits refer the following links: + [Email sending limits](https://support.google.com/a/answer/166852 Email sending limits) + [Error messages once limit is crossed](https://support.google.com/mail/answer/22839 Error messages once limit is crossed)

Settings to be updated on Google

1. Before sending emails using the Gmail\'s SMTP Server, Change the required settings under your Google Account Security Settings or click here.  
   
   
2. Make sure that 2-Step-Verification is disabled.  
   
   
3. Turn ON the Less Secure App access or Click here; https://myaccount.google.com/u/0/lesssecureapps.  
   
   
4. If 2-step-verification is enabled, then you will have to create app password for your application or device. 
   
   
5. For security measures, Google may require you to complete this additional step while signing-in. Click here to allow access to your Google account using the new device/app.  
   
   

Note: It may take an hour or more to reflect any security changes ## Writing the GO Code to Send Email using Gmail SMTP Server SMTP/NET package implements the Simple Mail Transfer Protocol as defined in RFC 5321.

func SendMail(addr string, a Auth, from string, to []string, msg []byte) error

Parameters + addr is a Host Server Address along with Port Number separated by Column \':\' + a is an Authentication response from Gmail + from is an Email Address using which we are authenticating and sending Email + to can a single Email Address or an array of Email Address to which we want to send an Email + msg - This parameter should be an RFC 822-style with headers first, a blank line, and then the message body. - The content should be CRLF terminated. - The headers part includes fields such as From, To, Subject, and Cc. - Sending Bcc messages is accomplished by including an email address in the to parameter but not including it in the msg headers. - This function and the net/SMTP package are low-level mechanisms and do not provide support for DKIM signing, MIME attachments and other features.

GO Code For Sending Email As HTML 

Click Here To Download The Complete Working Code For Sending Email As HTML. 

Explaining The Working Code: 

Step 1: Import required packages

• log :: log.Print() to print important stages and errors
• fmt :: fmt.Sprintf() To print formatted text
• net/smpt :: smtp.PlainAuth() is to authenticate account and smtp.SendMail() is to send Email using SMTP Protocol
• mime/quotedprintable :: quotedprintable.NewWriter() Is convert Email body in "Quoted Printable" Format. Click here To know more about the format.

package main
import (
    "log"
    "fmt"
    "bytes"
    "net/smtp"
    "mime/quotedprintable"
)

Step 2: Set required parameters to authenticating access to SMTP

from_email:= "from-email@domain"
password  := "gmail-app-password"
host      := "smtp.gmail.com:587"
auth      := smtp.PlainAuth("", from_email, password, "smtp.gmail.com")

Step 3: Set required Email header parameters like From, To and Subject

header := make(map[string]string)
to_email        := "recipient-email@domain"
header["From"]   = from_email
header["To"]     = to_email
header["Subject"]= "Write Your Subject Here"

Step 4: Set header parameters to define type of Email content.

header["MIME-Version"]              = "1.0"
header["Content-Type"]              = fmt.Sprintf("%s; charset=\"utf-8\"", "text/html")
header["Content-Disposition"]       = "inline"
header["Content-Transfer-Encoding"] = "quoted-printable"

Step 5: Prepare Formatted header string by looping all Header parameters.

header_message := ""
for key, value := range header {
    header_message += fmt.Sprintf("%s: %s\r\n", key, value)
}

Step 6: Prepare Quoted-Printable Email body.

body := "<h1>This is your HTML Body</h1>"
var body_message bytes.Buffer
temp := quotedprintable.NewWriter(&body_message)
temp.Write([]byte(body))
temp.Close()

Step 7: Prepare final Email message by concatenating header and body.

final_message := header_message + "\r\n" + body_message.String()

Step 8: Send Email and print log accordingly

status  := smtp.SendMail(host, auth, from_email, []string{to_email}, []byte(final_message))
if status != nil {
    log.Printf("Error from SMTP Server: %s", status)
}
log.Print("Email Sent Successfully")

package main
import (
    "log"
    "net/smtp"
)
func main() {
    to_email     := "recipient-email@domain"
    from_email   := "from-email@domain"
    password     := "gmail-app-password"
    subject_body := "Subject: Write Your Subject\n\n" + "This is your Email Body"
    status       := smtp.SendMail("smtp.gmail.com:587", smtp.PlainAuth("", from_email, password, "smtp.gmail.com"), from_email, []string{to_email}, []byte(subject_body))
    if status != nil {
        log.Printf("Error from SMTP Server: %s", status)
    }
    log.Print("Email Sent Successfully")
}

List of Possible Errors And Exceptions

Following are some of the errors which you may encounter while testing Gmail SMTP Module:

Error 1: If you have entered wrong credentials

2019/09/18 12:21:51 Error from SMTP Server: 535 5.7.8 Username and Password not accepted. Learn more at
5.7.8  https://support.google.com/mail/?p=BadCredentials c8sm5954072pfi.117 - gsmtp

Error 2: If you have not enabled App Password

2019/09/18 11:46:49 Error from SMTP Server: 534 5.7.9 Application-specific password required. Learn more at
5.7.9  https://support.google.com/mail/?p=InvalidSecondFactor s141sm5130851pfs.13 - gsmtp

Error 3: If you have entered wrong Email Address

2019/09/18 13:16:06 Error from SMTP Server: 553 5.1.2 The recipient address <recipient-email> is not a valid RFC-5321
5.1.2 address. w6sm8782758pfj.17 - gsmtp

You can also try package named Gomail for sending mail via Gmail.

Conclusion

Hope the steps explained above were useful and you were able to successfully send mail from your Gmail SMTP server using GO. Feel free to contribute, in case you encountered some issue which is not listed as a part of this tutorials. Use below comments section to ask/share any feedback.

<-- Happy Coding -->