Although I do like ProtonMail, the implementation of conversation is broken: receiving 2 emails from the same sender with the same subject is NOT a conversation...
Example: on day 1, LinkedIn sends an email with "You have 1 new invitation". 2 weeks later LinkedIn sends an email with "You have 1 new invitation". There is no connection between these 2 emails yet ProtonMail flags them as as conversation!
@protonmail Another example: my parents often send me email without any subject (because they don't really understand how email works...). ProtonMail keeps on lumping all these non related emails to the same conversation.