Bug: Content Type “multipart/alternative” emails failing

CURRENTLY

Offload SES Lite is failing to send emails of Content Type = “multipart/alternative”.

This was specifically found for WooCommerce sent emails on a Bitnami WordPress instance on AWS EC2. WooCommerce gives you the option to set the Content Type. When the email type = “multipart/alternative”, the email fails and is recorded as “Failed” in Offload SES Lite Activity.

There is nothing recorded in debug logs, but in phpmyadmin, in “failures” table the error text is as follows:

DeliciousBrains\WP_Offload_SES\Aws3\Aws\Ses\Exception\SesException : Error executing "SendRawEmail" on "https://email.ap-southeast-2.amazonaws.com"; AWS HTTP error: Client error: POST https://email.ap-southeast-2.amazonaws.com resulted in a 400 Bad Request response:
<ErrorResponse xmlns="https://ses.amazonaws.com/doc/2010-12-01/">
  <Error>
    <Type>Sender</Type>
    <Code>InvalidPara (truncated...)
 InvalidParameterValue (client): Missing start boundary - <ErrorResponse xmlns="https://ses.amazonaws.com/doc/2010-12-01/">
  <Error>
    <Type>Sender</Type>
    <Code>InvalidParameterValue</Code>
    <Message>Missing start boundary</Message>
  </Error>
  <RequestId>redacted</RequestId>
</ErrorResponse>

EXPECTED BEHAVIOUR

Emails set to Content Type = “multipart/alternative” do not fail.

WORKAROUND

Set emails to Content Type= “html”