From the Google Transparency Report
Many email providers don’t encrypt messages while they’re in transit. When you send or receive emails with one of these providers, these messages are as open to snoopers as a postcard in the mail.
A growing number of email providers are working to change that, by encrypting messages sent to and from our services using Transport Layer Security (TLS). When an email is encrypted in transit with TLS, it makes it harder for others to read what you’re sending. The data below explains the current state of email encryption in transit.
Full Article is here
Of course as you may have suspected your email may go through unsecured servers the article from Google bears that out. Never send sensitive/private information through email!