Add POSTFIX_STRIP_SENDER_HEADER Environment Variable

[AndrewPaglusch]

When inbound emails contain a "Sender" header, AnonAddy doesn't strip it off when forwarding the email. This causes problems if you've configured the application to relay emails via Amazon SES, since it validates that ALL email headers (From, Sender, and Return-Path) contain only verified identities in your SES account. If any of these headers contain an unverified domain, SES rejects the entire email with an error like:

554 Message rejected: Email address is not verified. The following identities failed the check in region US-WEST-2: no-reply@not-my-domain.com

This change adds a POSTFIX_STRIP_SENDER_HEADER env var (default false), which will optionally configure Postfix to strip off the "Sender" header from emails if present before attempting to relay them.

To enable Sender header stripping, set the environment variable:

POSTFIX_STRIP_SENDER_HEADER=true

Fixes #267
Related: anonaddy/anonaddy#471

[AndrewPaglusch]

I've just tested and verified this change works exactly as expected. Thank you to @buxm for providing the exact changes needed to resolve this issue.

Removing draft flag from this PR and opening it up for review..

[AndrewPaglusch]

Hey @crazy-max! Just wanted to give this a friendly bump – it's been about 7 weeks since submission. Would love to get your eyes on this when you have a chance. Thanks for all your work on the project!

[crazy-max]

Can you also add the default value for this env at

docker/rootfs/etc/cont-init.d/00-env

Lines 84 to 92 in d5210af

	POSTFIX_DEBUG=${POSTFIX_DEBUG:-false}
	POSTFIX_MESSAGE_SIZE_LIMIT=${POSTFIX_MESSAGE_SIZE_LIMIT:-26214400}
	POSTFIX_SMTPD_TLS=${POSTFIX_SMTPD_TLS:-false}
	POSTFIX_SMTP_TLS=${POSTFIX_SMTP_TLS:-false}
	POSTFIX_RELAYHOST_AUTH_ENABLE=${POSTFIX_RELAYHOST_AUTH_ENABLE:-false}
	POSTFIX_RELAYHOST_SSL_ENCRYPTION=${POSTFIX_RELAYHOST_SSL_ENCRYPTION:-false}
	#POSTFIX_SPAMHAUS_DQS_KEY=${POSTFIX_SPAMHAUS_DQS_KEY:-null}
	#POSTFIX_RELAYHOST_USERNAME=${POSTFIX_RELAYHOST_USERNAME:-null}
	#POSTFIX_RELAYHOST_PASSWORD=${POSTFIX_RELAYHOST_PASSWORD:-null}

[willbrowningme]

Looks good to me, thanks all.