K9 Mail error: Failed to send some messages - Negative SMTP reply: 553

K9 Mail error: Failed to send some messages - Negative SMTP reply: 553

I recently started getting the following error when trying to send an email on my android via K9 mail:
Failed to send some messages - Negative SMTP reply: 553

The issue happens only on my zoho mail account when using K9 Mail on Android. My gmail account seems to be fine (also on K9). No issues with my zoho mail account using Outlook or other mail clients.

The message in question gets moved into the Drafts folder. When I refresh the Draft folder - it vanishes without a trace.

I am logging this issue here as seems to be related to Zoho Mail account/K9 app combo

Any help is appreciated