We've become aware of a problem regarding our sent mail. I'm not sure how widespread it is since there is no read-receipt option in the email and many people do not follow up with us to let us know they didn't get a message. But we've learned that our messages are not being received by non-Zimbra users.

The message shows up in our sent box and we do not get an "undeliverable" message or any other message that would clue us into the problem. We only know about it if we message a client or call them to discuss the email and they say they never got it. This has happened to a variety of intended recipients and I'm not sure how long it has been going on. Some messages have attachments and some do not. I use Google Chrome, but my business partner uses Explorer.

Any solutions or ideas to ensure messages are being sent? Is there a way to get a "read receipt" in Zimbra mail? This has caused us to look like slackers with clients I'm afraid since we do not look responsive. And with all our traffic, I don't want to call every single recipient to see if they got a message. We are using 5.0.19.