Mailbox Keeps Accumulating Even When "Delete" Option Checked

Mailbox Keeps Accumulating Even When "Delete" Option Checked

I'm forwarding all mail from one of my inboxes to a Gmail account (no POP, IMAP, etc.). I've also set the mailbox to "Delete Zoho Mail's copy". However, the Zoho inbox keeps filling up, and I'm routinely getting emails that the mailbox is full. None of my other Zoho inboxes are experiencing this problem, even though the settings are identical. Do you know how I can resolve this? Thanks in advance!