removed MIME-Version, it gets duplicated

[sandropons]

it seems that Wordpress phpmailer already inserts MIME-Version in mail headers.
When "custom contact form" plugin sends the notification email, it adds the MIME-Version header and it gets duplicated.
Servers may not accept the mail, resulting the error: 554 Transaction failed: Duplicate header 'MIME-Version'.

[robbytx]

👍 I am encountering this same problem, sending through Amazon SES.

This seems to be a common problem for many Wordpress plugins, as a simple google search for "wordpress duplicate mime-version" shows.

[robbytx]

@tlovett1 is there any chance you would merge and release this in the near future?

PHPMailer in Wordpress Core clearly adds the same header (and appears to have done so for some time): https://github.com/WordPress/WordPress/blob/5eb452d15b4d99fcd19bc97dd53876547c7b2655/wp-includes/class-phpmailer.php#L2110

Also, the wp_mail docs have an example shown passing the Content-Type header without the MIME-Version header: https://developer.wordpress.org/reference/functions/wp_mail/#comment-348

Since the core wp_mail function always calls PHPMailer, this seems like a fairly straightforward change with a relatively low chance of regression. (Of course, I'm not a Wordpress plugin dev, so possibly I'm overlooking something.)