The Zimlet gallery is the best place to find and publish extensions for Zimbra.
Recalling/Undoing a Sent Message – UpMail
Zelda - Links Shield / Origami Rare Trading Card #A002 - Trading
Zeta Alliance - Undo Send Zimlet - delays email-send upto 30 seconds
Gundam Versus - PlayStation 4 : Bandai Namco Games
How to Recall or Cancel a Sent Email in Outlook
Zeta Alliance - Confidential header Zimlet
Zeta Alliance - Modern?page=2&sort=downloads Zimlets
How to use Undo Send
Messaging
LOU Mental Health Alliance Hopes to Bring Awareness Via Social
Unsend email with Undo Send in Mail on iPhone - Apple Support (CA)
Zeta Alliance - Undo Send