No, because ProtonMail scans all unencrypted e-mails and keeps private keys. 

 Look at OTP string - first 12 characters.
Familiarize yourself with YubiCloud too.
 

 Protonmail scans e-mails. 

 https://proton.me/legal/transparency 

 Account Activity: Due to limitations of the SMTP protocol, we have access to the following email metadata: sender and recipient email addresses, the IP address incoming messages originated from, attachment name, message subject, and message sent and received times. We do NOT have access to encrypted message content, but unencrypted messages sent from external providers to your Account, or from Proton Mail to external unencrypted email services, are scanned for spam and viruses to pursue the legitimate interest of protecting the integrity of our Services and users. Such inbound messages are scanned for spam in memory, and then encrypted and written to disk 

 😃 

 Protonmail has your private keys😃

And...

Due to limitations of the SMTP protocol, we have access to the following email metadata: sender and recipient email addresses, the IP address incoming messages originated from, attachment name, message subject, and message sent and received times. We do NOT have access to encrypted message content, but unencrypted messages sent from external providers to your Account, or from Proton Mail to external unencrypted email services, are scanned for spam and viruses to pursue the legitimate interest of protecting the integrity of our Services and users. Such inbound messages are scanned for spam in memory, and then encrypted and written to disk.  

 To be clear.
They scan both sent and received.


"from external providers to your Account, or from Proton Mail to external unencrypted email services"

 

 Yes, but many people assume that Protonmail is special.
This is mostly marketing gibberish.

Using GPG/PGP or DeltaChat with Gmail is more secure than using ProtonMail with external providers. 

 PGP encrypted with private keys stored in ProtonMail?
 

 Using Gmail with GPG/PGP or with Delta.chat is more secure than using Protonmail or Tutanota. 

 I don't use it. I wrote that Gmail with Delta Chat or GPG was more secure that Protonmail or Tutanota. 

 I don't know what are you asking for.

Why is it more secure using Gmail +  GPG or Delta.Chat ?
Your keys your data.

Protonmail scans your emails and possesses your private keys.

Why I don't use Gmail?

I don't like it. 

 Start using GPG or Delta Chat 

 xmpp?