my settings are such that I get an email when someone replies to one of my messages. this is separate from the daily summaries.
normally, I get an email titled “[iNaturalist] Message Title” and in the text is “userA sent you a new message on iNaturalist”. further replies by userA under that topic create a new email that stacks/is under the original email.
However, say I send three messages to userB, userC, and userD, and each message title is “why spiders are cute”. When userB replies, I get the usual email notification, no problem. But then if userC and/or userD reply, I don’t get a separate email – instead, it’s sent as part of userB’s thread, so that one multipart email has messages from multiple, separate conversations.