We have two Admin's set for $admin_email, but this seems to cause an issue as it means any 'sent' email has two 'from' addresses listed.
Some email clients will not accept email with multiple 'from' addresses.
Is there a way around this, without deleting one of the Admin email addresses?

Steve

If it's not set yet, try to specify $reply_to_email option in setup_options.php (the Manual describes how to). There you have to specify just a single email address.

Ok, as I'm dumb, how do I set that?
It seems to be set to $admin_email, so do I just replace that text?

Steve

Don't replace it. Just paste as shown in the Manual (right after the $admin_email option), but removing // (two slashes) in front.

Slashes mean this setting is inactive and not used.

Set it like this:

$admin_email = 'email1, email2';
$reply_to_email='email3';

I'd make sure the "From" email is from the website's domain (otherwise it will surely go to spam or won't be delivered). And would make use of the $reply_to_email indeed, but like this:

$admin_email='support@example.com'; // example.com is your main domain
$reply_to_email='admin1@something.com, admin2@something2.net';

tom322 is quite right. :-) If the website in question has cPanel or some other control panel, perhaps there's a tool for email forwarders. That forward tool could be used to deliver mail pointed to A so it ends in B and C.