I got inspired. Here is how email works with encryption, assuming that VPN is not in use.
One thing to note is that email is clear (readable) text at the points noted in this diagram. It will be clear text at specific places.
The email is clear text when it travels from one email provider such as Verizon to another such as Google. But this is the domain of the internet backbone and I am fairly certain that you'd have to work at an ISP in order to sniff this level of internet traffic, since this is where your data flows with thousands or millions of other users.
The email is also clear text in the hands of the source as well as the destination email provider.
So, you have to trust email providers an awful lot when using normal email systems.
Protonmail would result in the message never being clear text when it leaves your workstation and only becomes clear text to the eyes of the recipient.
+--------------------+
| |
| Email User Sender| *** Original message in clear text ***
| Sends Message |
| |
| (workstation or |
| mobile phone's |
| email client |
| software) |
+--------------------+
|
| SMTP with SSL encryption flowing
| over the hotel WIFI and out to the
| email service of the user's choice
|
| If you use a VPN this is the ONLY
| level affected.
|
+--------------------+
| Web |
| Email (Verizon, |
| Yahoo, Gmail,etc)|
| |
+--------------------+
|
| SMTP clear text, no encryption
| The intertubes/public backbone
| The domain of ISP servers
| Not accessible by normies,
| commoners, hotel guests, etc.
| *** Original message in clear text ***
|
+--------------------+
| Web |
| Email (Verizon, |
| Yahoo, Gmail,etc)|
| SMTP server |
+--------------------+
|
| Mail queue, with a server such
| as Dovecot that provides
| email on request of POP3 server.
| (internal to email servers, accessible
| by employees, may or may not be encrypted)
| *** Original message in clear text ***
|
+------------------------+
| |
| POP3 Server |
| Internal to email |
| service provider |
+------------------------+
|
| POP3 probably with SSL encryption flowing
| over the whatever networking that the
| recipient is using
|
+------------------------+
| |
| Email User Recipient |
| Receives Message | *** Original message in clear text ***
| |
+------------------------+