AWS Lambda SES Email Forwarder

A Node.js script for AWS Lambda that uses the inbound/outbound capabilities of AWS Simple Email Service (SES) to run a "serverless" email forwarding service.

Instead of setting up an email server on an EC2 instance to handle email redirects, use SES to receive email, and the included Lambda script to process it and send it on to the chosen destination.

Limitations

SES only allows sending email from addresses or domains that are verified. Since this script is meant to allow forwarding email from any sender, the message is modified to allow forwarding through SES and reflect the original sender. This script adds a Reply-To header with the original sender, but the From header is changed to display the original sender but to be sent from the original destination.

For example, if an email sent by Jane Example <jane@example.com> to info@example.com is processed by this script, the From and Reply-To headers will be set to:
```
From: Jane Example at jane@example.com <info@example.com>
Reply-To: jane@example.com
```
To override this behavior, set a verified fromEmail address (e.g., noreply@example.com) in the config object and the header will look like this.
```
From: Jane Example <noreply@example.com>
Reply-To: jane@example.com
```
SES only allows receiving email sent to addresses within verified domains. For more information, see: http://docs.aws.amazon.com/ses/latest/DeveloperGuide/verify-domains.html
SES only allows sending emails up to 10 MB in size (including attachments after encoding). See: https://docs.aws.amazon.com/ses/latest/DeveloperGuide/limits.html
Initially SES users are in a sandbox environment that has a number of limitations. See: http://docs.aws.amazon.com/ses/latest/DeveloperGuide/limits.html

Set Up

Modify the values in the config object at the top of index.js to specify the S3 bucket and object prefix for locating emails stored by SES. Also provide the email forwarding mapping from original destinations to new destination.
In AWS Lambda, add a new function and skip selecting a blueprint.

Name the function "SesForwarder" and optionally give it a description. Ensure Runtime is set to Node.js v8.10.
For the Lambda function code, either copy and paste the contents of index.js into the inline code editor or zip the contents of the repository and upload them directly or via S3.
Ensure Handler is set to index.handler.
For Role, choose "Basic Execution Role" under Create New Role. In the popup, give the role a name (e.g., LambdaSesForwarder). Configure the role policy to the following:

{
   "Version": "2012-10-17",
   "Statement": [
      {
         "Effect": "Allow",
         "Action": [
            "logs:CreateLogGroup",
            "logs:CreateLogStream",
            "logs:PutLogEvents"
         ],
         "Resource": "arn:aws:logs:*:*:*"
      },
      {
         "Effect": "Allow",
         "Action": "ses:SendRawEmail",
         "Resource": "*"
      },
      {
         "Effect": "Allow",
         "Action": [
            "s3:GetObject",
            "s3:PutObject"
         ],
         "Resource": "arn:aws:s3:::S3-BUCKET-NAME/*"
      }
   ]
}

Configure the Memory and Timeout settings according to your use case. For simple text emails, a memory limit of 128 MB and timeout of 10 seconds should be sufficient. For emails with large attachments, a memory limit of 512 MB or more and timeout of 30 seconds may be required.

In AWS SES, verify the domains for which you want to receive and forward email. Also configure the DNS MX record for these domains to point to the email receiving (or inbound) SES endpoint. See SES documentation for the email receiving endpoints in each region.
If you have the sandbox level of access to SES, then also verify any email addresses to which you want to forward email that are not on verified domains.
If you have not configured inbound email handling, create a new Rule Set. Otherwise, you can use an existing one.
Create a rule for handling email forwarding functionality.

On the Recipients configuration page, add any email addresses from which you want to forward email.
On the Actions configuration page, add an S3 action first and then an Lambda action.
For the S3 action: Create or choose an existing S3 bucket. Optionally, add an object key prefix. Leave Encrypt Message unchecked and SNS Topic set to [none].
For the Lambda action: Choose the SesForwarder Lambda function. Leave Invocation Type set to Event and SNS Topic set to [none].
Finish by naming the rule, ensuring it's enabled and that spam and virus checking are used.
If you get an error like "Could not write to bucket", follow step 7 before completing this one
If you are asked by SES to add permissions to access lambda:InvokeFunction, agree to it.

The S3 bucket policy needs to be configured so that your IAM user has read and write access to the S3 bucket. When you set up the S3 action in SES, it may add a bucket policy statement that denies all users other than root access to get objects. This causes access issues from the Lambda script, so you will likely need to adjust the bucket policy statement with one like this:

{
   "Version": "2012-10-17",
   "Statement": [
      {
         "Sid": "GiveSESPermissionToWriteEmail",
         "Effect": "Allow",
         "Principal": {
            "Service": "ses.amazonaws.com"
         },
         "Action": "s3:PutObject",
         "Resource": "arn:aws:s3:::S3-BUCKET-NAME/*",
         "Condition": {
            "StringEquals": {
               "aws:Referer": "AWS-ACCOUNT-ID"
            }
         }
      }
   ]
}

Optionally set the S3 lifecycle for this bucket to delete/expire objects after a few days to clean up the saved emails.

Extending

By loading aws-lambda-ses-forwarder as a module in a Lambda script, you can override the default config settings, change the order in which to process tasks, and add functions as custom tasks.

The overrides object may have the following keys:

config: An object that defines the S3 storage location and mapping for email forwarding.
log: A function that accepts log messages for reporting. By default, this is set to console.log.
steps: An array of functions that should be executed to process and forward the email. See index.js for the default set of steps.

See example for how to provide configuration as overrides.

Troubleshooting

Test the configuration by sending emails to recipient addresses.

If you receive a bounce from AWS with the message "This message could not be delivered due to a recipient error.", then the rules could not be executed. Check the configuration of the rules.
Check if you find an object associated with the message in the S3 bucket.
If your Lambda function encounters an error it will be logged in CloudWatch. Click on "Logs" in the CloudWatch menu, and you should find a log group for the Lambda function.

Credits