Mailbird does not have the ability to mark an email as read or delete them automatically. If your email is marked as read automatically, or deleted, this means that the email in your server is marked as read by some other email client. Mailbird is simply syncing to the server.
The most common reason for this is if you set up your email as IMAP in Mailbird and also set it up as POP3 in other email clients. When you do this, once your other email clients download the email from your server, it will then delete it from the server. Since Mailbird is using IMAP it will always sync to the email server and will also delete the email from Mailbird. Which is why it might seems missing.
We cannot really tell you which of your email clients is doing this, but you can use a simple elimination process to find out which one is causing the issue.
Simply disable all your email clients except for Mailbird, including your mobile/tablet email client. Then try turning them on one by one. If the issue suddenly appear, you will now which email client is causing the issue.
If you have this issue go to your email provider webmail and check if the email is still there. If it's not there then Mailbird is already working correctly. If it's there, please contact us at firstname.lastname@example.org