Unfortunately it's not possible for Squarespace to change your domain settings, as we don't host this. When you map your domain to Squarespace, the CNAME points the 'www' to us, but we don't offer e-mail services, so your MX records (which handle your e-mail) should not point to us.
To receive email on your domain, there should be two records set up similar to the following:
yourdomain.com MX mail.yourdomain.com
These two records first create a named system that's responsible for your email, then create a record that forwards your email through that system. Your provider's email server IP is going to vary per provider -- and you should contact your provider to ensure everything is configured properly.