BuddyPress.org

If you’ve already tried the solutions in this thread:

BuddyPress 3.1.0 is now available!

Then, here are a couple of other possible solutions:

1) Remove the “Reply-To” email address from BuddyPress emails.

By default, BuddyPress adds a “Reply-To” email header to each BuddyPress email containing the admin user’s email address.

I’m guessing email senders might have problems with this because the “From” email address and the “Reply-To” email address could use different domains. Email senders might think this is spammy and would reject the email. Just a guess. You’d have to look in your email logs to confirm.

Anyway, here’s a code snippet you can try in your theme’s functions.php or in wp-content/bp-custom.php:

add_filter( 'bp_email_set_reply_to', function( $retval, $email_address ) {
	// Wipe out Reply-to email header if it matches the admin email address.
	if ( bp_get_option( 'admin_email' ) === $email_address ) {
		$retval = new BP_Email_Recipient( '' );
	}

	return $retval;
}, 10, 2 );

2) Try setting a different “From” email address

If you are having problems sending any email on your WordPress install, you might have to set the “From” email address to one that contains your website domain.

There are a few plugins that can do this. Here are a couple:

Change From Email

WP Change Default From Email

3) Try using a WordPress email SMTP plugin

If all the above fail, try a SMTP email plugin:
https://wordpress.org/plugins/tags/smtp/

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Please let me know if either of the solutions work.