Issues Sending Emails to Proton Mail Addresses

About the privacy aspect, Proton Mail does have strong encryption, but this issue is likely not related to their security measures. It seems more about whether the email addresses are valid. Remember that their free tier might attract spammers, which could contribute to this misunderstanding.

You might want to check if your users can use alternative domains like pm.me or protonmail.com to send their emails instead. Sometimes using a different domain can resolve these kinds of issues.

It's also a good idea to consider whether these clients are legitimate. Are they new contacts, or do they have an established history with your users? If it's an old mailing list, there might be defunct emails included.

Make sure the email addresses are correct. Have the users double-check by calling their contacts and spelling the addresses over the phone. That can help rule out simple mistakes.

Since you're getting a "User not found" error, it's likely that the issue isn't on your end. Proton Mail is indicating that those specific users don't exist. Just to be thorough, you could create a test Proton account to see if emails go through.