This guy called me yesterday. I explained my theory about port 25 not working all the time. He did some testing. His reply below. I think he's wrong but I need you to verify that.
After reviewing the errors from testing your contact forms, this appears to be an issue with the way you are sending the messages.
You are using sendmail instead of SMTP, so this doesn't have anything to do with ports.
The problem is that you are using the email address from the contact form as the "from" address which gets rejected since you're in effect spoofing that address.
To make this more obvious, I've temporarily updated the destination email address in contact.php to email@example.com and then filled out the contact form with firstname.lastname@example.org in the email field.
Your form attempted to send a message as if it's from email@example.com, which was, of course, rejected by gmail because this violates Yahoo's DMARC policy:
2014-07-28 17:13:58 1XBtB8-00044L-Jt <= firstname.lastname@example.org U=classicp P=local S=1015 email@example.com T="Contact Classic Pumps" from <firstname.lastname@example.org> for email@example.com
2014-07-28 17:13:59 cwd=/var/spool/exim 3 args: /usr/sbin/exim -Mc 1XBtB8-00044L-Jt
2014-07-28 17:13:59 1XBtB8-00044L-Jt SMTP connection outbound 1406585639 1XBtB8-00044L-Jt classicpumps.com firstname.lastname@example.org
2014-07-28 17:14:01 1XBtB8-00044L-Jt ** email@example.com F=<firstname.lastname@example.org> R=autorespond_local_smtp T=remote_smtp: SMTP error from remote mail server after end of data: host gmail-smtp-in.l.google.com [18.104.22.168]: 550-5.7.1 Unauthenticated email from yahoo.com is not accepted due to domain's\n550-5.7.1 DMARC policy. Please contact administrator of yahoo.com domain if\n550-5.7.1 this was a legitimate mail. Please visit\n550-5.7.1 http://support.google.com/mail/answer/2451690 to learn about DMARC\n550 5.7.1 initiative. f20si44211830icc.101 - gsmtp
2014-07-28 17:14:01 cwd=/var/spool/exim 7 args: /usr/sbin/exim -t -oem -oi -f <> -E1XBtB8-00044L-Jt
2014-07-28 17:14:02 1XBtBC-00048e-05 <= <> R=1XBtB8-00044L-Jt U=mailnull P=local S=2340 T="Mail delivery failed: returning message to sender" from <> for email@example.com
2014-07-28 17:14:02 1XBtB8-00044L-Jt Completed
Here are some possible solutions:
1. Use SMTP instead of mail() / sendmail with a valid account at the domain the contact form resides on.
2. Use the cPanel account's domain in the From header and the contact form email address in the Reply-to header
If you have any further questions or require additional assistance, please don't hesitate to reply and we'll be happy to help.
Linux Systems Administrator