It depends on the design of a particular service, but if you send an email outside of a service providing encrypted emails it is subject to the same relaying mechanisms as every other email on the Internet.
This means you don't know and have no control over (and that's reason you want to avoid sending unencrypted emails in the first place).
Regarding ProtonMail it offers two solutions for encrypted communication with recipients outside the service (source Wikipedia, March 2016):
Emails sent from ProtonMail to non-ProtonMail email addresses may be sent with or without encryption. With encryption, the email is encrypted with AES under a user-supplied password and then stored on ProtonMail's servers. The recipient receives a link to the ProtonMail website on which they can enter the password and read the decrypted email.
(...) ProtonMail added native support to their web interface and mobile app for Pretty Good Privacy (PGP). This allows a user to export their ProtonMail PGP-encoded public key to others outside of ProtonMail, enabling them to use the key for email encryption.
In practice a test email (with both encrypted and unencrypted content) from
@gmail.com email was sent directly between the two parties using encrypted and authenticated protocol ESMTP with STARTTLS:
Received: from mail2.protonmail.ch (mail2.protonmail.ch. [22.214.171.124])
by mx.google.com with ESMTPS id e3si15787607wjn.27.2016.03.18.05.07.18
(version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128);
Fri, 18 Mar 2016 05:07:18 -0700 (PDT)
spf=pass (google.com: domain of email@example.com designates 126.96.36.199 as permitted sender) firstname.lastname@example.org;
dmarc=pass (p=QUARANTINE dis=NONE) header.from=protonmail.com
This post also touches on the subject of encrypted email transmission.