554 5.2.3 mailpolicy violation error

554 5.2.3 mailpolicy violation error

I have about 40 clients on Zoho. Today I emailed one of them and got this error. I looked it up and found it has to do with too large an attachment. I sent no attachment. So I'm guessing this may have to do with her mailbox full or someone else sent too large an attachment and now everyone's email is blocked. Can you please look into this? The email address is susan@susanhowlett.com. Thanks.