Unable to delete emails from outlook mailbox

I'm having the same problem with a new customer.

We're migrating their old emails from the old IMAP server to 365 Business. Problem is, the old mailbox has 350GB data. So I'm forced to use Thunderbird, since Outlook PST - after changing registry to allow unlimited size instead of 50GB - breaks over 100-120GB size. Basically, I download a bunch of mails with Thunderbird (20-30GB in a go), then upload them with via IMAP into 365, then load Outlook Classic (Exchange) and move them from the licensed mailbox into a shared (archive) mailbox.

And I'm regularly getting these undeletable mails now. Apparently something breaks when messing via IMAP or so many (hundreds of thousands) of emails or Thunderbird.

I tried everything and the solutions didn't work so now I'm using a temp 365 mailbox with a Business Basic license for the process, and any time it breaks - getting 10-20-30GB undeletable mails eventually - I just delete the entire M365 user account and recreate it fresh, re-add the license and continue.

Pro tip: do not ever put 350GB email in a linux-based IMAP mailbox because it will break everything.

Tried:

• Deleting from Outlook Classic (Exchange cached and online mode)
• Deleting from Outlook Classic (IMAP)
• Deleting from Thunderbird (IMAP)
• Deleting from OWA
• Deleting from OWA Settings - Account - Storage

Funny, but I just had one such batch of 15GB / 8000 emails, where I could delete half of them via OWA but the rest, they just won't go away.

These e-mails are only visible in OWA, IMAP or Exchange client does not show them.

Also, Thunderbird randomly stops processing the upload after 100-4000 emails.

Just... don't do this. Please. If your (old) admin says it'll be fine, run.

Lo siento, no hablo español, este es el post de arriba traducido en Google.

Desde mediados de 2023, Microsoft ha trasladado gradualmente el almacenamiento de Outlook a OneDrive, reduciendo la capacidad para archivos personales. Esta es la ruta que causa su problema. Al parecer, se lo dijeron a todos, ¡pero no lo recuerdo!

Para desconectar OneDrive (mejor en una PC/portátil), descargué todos mis archivos/carpetas a mi PC.

Entonces, hice esto:

Abra la configuración de OneDrive (seleccione el ícono de la nube de OneDrive en su área de notificación, seleccione el ícono de Ayuda y configuración de OneDrive, luego Configuración).

Vaya a la pestaña Cuenta.

Seleccione Desvincular esta PC.

Ahora, hago una copia de seguridad en una unidad externa y uso Google Drive para mis carpetas/archivos que quiero compartir entre dispositivos, PERO eso debe hacerse manualmente.

Hago una copia de seguridad de mis dispositivos móviles en la nube del fabricante, en mi caso Samsung.

Confía en que esto te ayuda.

Hi Patsy,

Thanks for your comment.

Are you using the new Outlook? If so, Microsoft is still developing it. I found this out after going directly to Support, an Agent took over my PC and set Outlook 365 up for me in Microsoft 365 (formerly Office 365).

Microsoft is not advising users of changes to the new Outlook, hence you will have surprise issues.

If you don't have 365, go back to the Classic Outlook, it is stable.

This article may be of help: Choose the right version of Outlook - Microsoft Support

I have lost hundreds of photos and cannot delete emails etc

Help

Basically what it says above