smtp2zoho

smtp2zoho works as a proxy, receiving SMTP messages, forwarding mails to Zoho Mail API.

It is usually used to hide sender's IP address when using SMTP.

Purpose

In other words, it is a middleware between an app that sending mails via SMTP and Zoho Mail API, "updating" the app to support Zoho Mail API silently and non-invasively.

Undoubtedly, Zoho has been providing native SMTP access for all users. However, It is a problem that sending mail via SMTP could expose the IP address of sender. Receiver could check mail headers (it is easy as you want), which contains Received header, indicating sender IP address. When you are sending a mail by web app or HTTP API, IP of sender would not contained in headers. That is what it based on to avoid IP exposure.

Configure

Configure options are contained in "config.yml". Deploy your own Zoho access credential and specify fake SMTP server listen address.

Create your Zoho client

First, create a "Self Client".

Setup OAuth with Zoho > Self Client

Get authorization code with this scope: "ZohoMail.accounts.READ,ZohoMail.messages.CREATE".

Kaizen #2 - OAuth2.0 and Self Client #API

Now your have Client ID, Client Secret and Authorization Code.

Get OAuth Token

Request this API to get Access Token and Refresh Token. (doc: https://www.zoho.com/mail/help/api/using-oauth-2.html)

POST /oauth/v2/token
Host: accounts.zoho.com
Content-Type: application/x-www-form-urlencoded

form data:

client_id: <ClientID>
client_secret: <ClientSecret>
grant_type: authorization_code
code: <AuthorizationCode>

It will response with Access Token and Refresh Token.

Get Account ID

Request this API to get account ID. (doc: https://www.zoho.com/mail/help/api/get-user-account-details.html)

GET /api/accounts
Host: mail.zoho.com
Authorization: Bearer <AccessToken>

It will response with user info, including Account ID (field name is "accountId").

Fill into config

Fill the required credentials into "config.yml". The field "ZohoMailAddr" is the sender's mail address. Understand that passing sender address in SMTP context with "MAIL FROM" command is just used to make a standard SMTP process. It won't be contained in HTTP request.

deploy SMTP listen address

Update the field "smtpListen".

Run server

Run it as other Go programs. No command arguments are required.

Worth Noticing

• DO NOT expose the SMTP server in public network. It is weak, non-standard SMTP session could make it panicked. And currently, there's no authentication option provided.
• If you want to specify sender display name, use From header in raw mail data. Here are some examples:

# normal
From: Specified Name <[email protected]>
# UTF-8 encoded
From: =?UTF-8?B?5Y+R6YCB6ICF?= <[email protected]>
# if you don't want to specify it, just leave it blank
From: <[email protected]>

• Subject header only allow formats below:

Subject: TestSubject
Subject: =?UTF-8?B?VGVzdEhlYWRlcg==?=

• Make issues or pull requests if you want to improve the project.