This won't happen for all emails. It depends on the TO address email server and whether they are checking SPF and blocking emails that don't come from an approved source. Nothing has changed on your site to cause this error. It is increased security on the site the email is being sent to.
I have updated the admin so that you can set the FROM and REPLY TO fields for the email separately. This will allow you to change the from and still allow users to reply.
I could help you with better instructions on how to set the SPF record as well. Who is your hosting provider for the olynet.com domain? I can google how to set a DNS TXT record for that hosting provider and send you a link.