I have yet to run into that issue.
There are a LOT of other factors in an email that
can affect deliverability like that. Stuff like use of certain words or punctuation in certain ways can do it. Same goes for pictures and certain types of links.
One thing worth considering with them is a paid account plus using a
custom email domain. Buy a web domain for usually ~$8-20/yr (
Namecheap is a good option), and then you can connect it with ProtonMail to create your own branded email. The "Plus" plan is $48/yr, which allows this. Why might you want to do this? Being that it's a separate domain from the standard protonmail.whatever domains, it's a clean slate to send from. Also, for anyone that has even a small business, looks much more professional.
Additionally, on their "Professional" plan ($75/yr), it gives you the capability for a
catch-all email. What this enables you to do is effectively create unlimited "fake" emails for various purposes that can have the side benefit of increasing security. For example, creating a separate "fake" email for EVERY single online account you create. These wouldn't be separate emails with inboxes, and would instead reroute to your catch-all account. For example, these would be valid emails:
HTML Code:
example@mydomain.com
example+1@mydomain.com
example+2@mydomain.com
example+3@mydomain.com
example+4@mydomain.com
Imagine the first is your catch-all. Each account you create could have a fake email like the numbered examples (many other valid ways to write those, so doesn't have to be that format). Any email sent to them (like forum notifications, for example) would route to the catch-all account. Now, if anyone tries to hack any of your accounts, if every single one is a different email and password, you have defense in depth.
Just a few random things hopefully people find useful.