If you are looking for a solution to save ProtonMail emails then I would recommend you to use Mail Backup Tool on your system. The software can directly download emails from ProtonMail to PC. With this utility, you can backup ProtonMail emails with attachments. The tool has multiple saving options that allow you to download emails into various formats.
Additionally, it provides various advanced filter options for selective backup. It preserves data integrity and email structure as well during the backup process.
Steps to Save ProtonMail emails with attachments
1. Run Mail Backup Tool on your system.
2. Choose ProtonMail and enter its login details to sign in.
3. select required mailbox folders.
4. Choose a saving option.
5. Browse target location and click on Backup.
Done!