When I am typing more than 10 characters in the subject field I get the following error message:
"Could not instantiate mail function."

Could you help me please...???

The site link is www.topsign.gr and press "contact"

Thank you!

There is no connection between the number of chars in the subject and the error you mentioned.
I'm guessing you don't have correctly configured an email server in your global configuration.
This article might help:
www.cocut.cn/tips/48-tips-and-tricks/323...omla-gmail-smtp.html

Dear after many many trials I have figured out that tha problem in the 'subject' field comes when I use greek fonts...
Whenever I insert latin characters I can use as many as i want...
When i use greek fonts I can insert up to 18 characters.
Can you advice me anything about that...???
Thank you!

I have tried Joomla's default contact page and I get the same error in the 'subject' field.
So it has nothing to do with your component.

Besides any help would be highly appreciated! You have sent me details of how to setup gmail account. I am not using gmail account anymore, but the domains mail.

Thank you!

I think you need to contact your hosting provider about this. The settings on the email server might be generating your problem.