content: Between Protonmail and Protonmail it may work. But the issues here are between ProtonMail and other mails. I think it is related to bad networks. It is not normal that image is sent through ProtonMail Android app and it ends up delivered without the attachment but with the text. I remember that up until end of 2017 ago it worked pretty stable but last few months many missed mails that are indicated a sent, but never delivered. The email is correct because I get a reply on the first message and any other replies I now send end up into a black hole. It is in my ProtonMail account sent box but never delivered.  Also not in the recipients spam box. It is no point in being secure if randomly fails te deliver or receive.