问题
So our system is sending us email notifications regarding specific incidents. The problem is, things get really spammy and disorganized, and we really want to have our email client (Gmail) thread notifications regarding the same incident.
So for instance:
Email 1 is sent about incident 1. Email 2 is sent about incident 1.
Gmail displays them in their own threads while we want them to appear as "replies" in the same thread.
I thought all I needed was to append "Re: Subject Line" but that doesn't work. Then I read about the References and In-Reply-To headers, but these require storing message IDs and we'd want to avoid the overhead if we can.
Is there any workaround to have clients thread emails received about the same topic without having to store message IDs and add them to In-Reply-To headers?
Thank you so much!
Note: Using PHPMailer
回答1:
You really can't get around this. You must have something that uniquely identifies a message, and as far as email messages go, that something is the message ID, and that's also exactly the thing referenced by References and In-Reply-To headers, so you really can't get away from that. It has to be implemented using message headers because that's universally supported by all email clients.
The definitive threading algorithm, is, like email itself, now very old, but you can find it here, and it's still as valid as it ever was. I used that approach to implement the email threading used in the chamsocial.com social network.
Some email clients try to do threading without using message IDs, and it's almost always a failure - I sometimes see Apple Mail doing this when IDs are missing, pulling completely unrelated messages into a thread just because they happen to have the same subject.
If you wanted to do message threading without message IDs, you'd need to think up something that acts in exactly the same way - which is pointless, so you're best off biting the bullet and implementing message ID support properly.
来源：https://stackoverflow.com/questions/61058551/how-to-send-reply-on-email-thread-using-phpmailer