site email contact form not sending to certain email addresses

Hello, a client is using CPanel and his zencart contact form will not send to his main domain email address, although when changing the email address in the "From" field to another email, his contact form works.

We have tried numerous email transport methods and also about 7 different email addresses into the "From" email address field. 3 emails seem to work fine but the others (all from different servers) seem to send mail, but nothing comes through to those email addresses.

We have no spam filters on these account and think this may be a CPanel or email hosting issue, can anyone else think of why emails from the zencart contact form would work under some addresses and not others (which we know are working email addresses)?

Many thanks for any ideas,
Al

internal server mail routing can be complicated. Sometimes a misconfiguration will cause mail "from" the server going "to" the same server to be seen as undesired.
Usually the best way around this is to use the 'sendmail -f' option.
There's a bug in 1.3.0.x that prevents sendmail -f from actually running. See below.

Another method for working around the internal routing problem is to enable the "Emails must send from known domain" option. This will cause the "sent from" email address to be set to match your "Email From" address set in Email Options.

To work around the sendmail -f bug so that you can try to use that option, do this:
/includes/functions/functions_email.php
around line 157:```
// if mailserver requires that all outgoing mail must go "from" an email address matching domain on server, set it to store address
if (EMAIL_SEND_MUST_BE_STORE=='Yes') $mail->From = EMAIL_FROM;

change to:```
      // if mailserver requires that all outgoing mail must go "from" an email address matching domain on server, set it to store address
      if (EMAIL_SEND_MUST_BE_STORE=='Yes') $mail->From = EMAIL_FROM;

      if (EMAIL_TRANSPORT=='sendmail-f' || EMAIL_SEND_MUST_BE_STORE=='Yes') {
        $mail->Sender = EMAIL_FROM;
      }

This is fixed in the next release.