simpleContact Pro sends emails to admin with the visitor's email address as the 'from' address. The SMTP server may not be convinced enough by this (because it is sending an email *from* yourdomain.com *claiming* to be from the user's email domain e.g. googlemail.com). Most ISPs have no problem with this but some are stricter. Here are a couple of suggestions:
Ask your host if you can enable SPF "Sender Policy Framework"
It may be worth trying to contacting your host to see if you can get SPF set up. I've had a previous instance before when this was necessary.
Hardcode the sending email address
You can also try hardcoding the sending email address to your admin email address:
Open sc_admin/include/contact.php
Change line 275 to this (replace "you@yourdomain.com" with your email address):
Please note: The code below should all be on one line.
sendMessage($recipients[$a],"you@yourdomain.com",$name_full,$email,
"A visitor has sent a message",$recipient_message);