unexpected bounce back

unexpected bounce back

Hello, I have both the MX records and the TXT records set up for the domain fwlab.com.
The content of the TXT record is " v=spf1 mx include:zoho.com ~all"
The first MX record, with priority 20 with name fwlab.com points to mx2.zohomail.com
The second MX record, with priority 10 with name fwlab.com points to mx.zohomail.com

That said, this configuration seems (to me) to be correct and has been working correctly for a while.
I have recently getting buonce back of emails sent by one of the email users, as if the TXT record was not correct.
" ERROR_CODE :550, ERROR_CODE :Verification failed for < stefania.cara@fwlab.com
            No Such User Here" 
            Sender verify failed"

- Do I need to change anything in our DNS? Was the TXT record in need of an update?

- Is there a transitory issue with zoho mail?

Best regards and thanks,
   Michele Fadda ( michele.fadda@fwlab.com