I have a board set up to send notifications via email when a card has been updated (for client support requests).
The email notification is received correctly.
However, on the email there is a link for "Reply via email" and when clicked in a desktop email client or mobile client the email is blank - it looks like you just opened a new email to send which is not helpful at all. When opened directly in a Gmail account (I didn't try with others) it will populate the reply email address.
Why does the reply button not work if you are outside Gmail? And why is it even there if it doesn't work? I would much prefer that to just not be shown and force the client to login to the board, but when they see the Reply link automatically click it then ask me why it doesn't work.
You can use Hipporello service desk for this purpose. With Hipporello Service Desk, you can build forms and you clieants can submit support tickets on your service desk portal provided by Hipporello. You can connect your support emails as well. In this way, you can centralize the communication with your customers in a single Trello card. You can reply the customer without going to your inbox and you and your customer can both reply directly from email. You can check https://www.hipporello.com/service-desk for the details.
I gave it a test on my side, and I can confirm those links are generic mailto: links, so their behavior would depend a bit on the default program the user has set to open them.
It's working in Gmail for me, both in OSX, iOS, and Android. I don't have a desktop email client to test, but that may be the culprit!
Those links are designed to populate the email field with the board's email-to-board address, and the subject field with the name of the card that generated that email notification.
We'd be glad to dig in a bit more with you if you'd like to create support ticket through trello.com/contact - we'll keep an eye out for your reply there!