Do not include Bcc header in delivered email

[adamu]

Proposal following this discussion.

According to RFC 5322 method 1:

when a message containing a "Bcc:" field is prepared to be sent, the "Bcc:" line is removed even though all of the recipients (including those specified in the "Bcc:" field) are sent a copy of the message

This PR removes the BCC header from the message that is delivered.

Note that the BCC addresses are still included in the recipients, so they will still receive the email. It is only the body that does not contain the bcc addresses:

swoosh/lib/swoosh/adapters/smtp.ex

Lines 55 to 59 in 3cab4d8

	recipients = all_recipients(email)
	body = Helpers.body(email, config)
	case :gen_smtp_client.send_blocking(
	{sender, recipients, body},

[princemaple]

💜

[adamu]

Digging some more, it looks like this was deliberately added in #105. There might be multiple use-cases? At least the comment I left explains why it's not there now.

[princemaple]

That didn't make sense :) I bet they just thought it was not sending correctly.

[thiamsantos]

@princemaple Hello, this change broke our implementation after the update, we had to rollback to v1.11.0.

We use the adapter Swoosh.Adapters.AmazonSES, one of our emails do not have any to but only the bcc fields. I imagine that as the adapter uses the smtp helper to build a email and send via http to aws, SES stopped receiving the bcc and cannot build the receipients.

We started receiving from aws the following error:

{:error, %{code: "InvalidParameterValue", message: "Missing required header 'To'."}}

[adamu]

It looks like this could be the case for the Sendmail adapter too. I guess they should not be using the SMTP Adapter's internal helper code.

[princemaple]

@thiamsantos looks like we have to distinguish between adapters that use SMTP format and real SMTP on handling of BCC. Thanks for reporting. Will fix soon.