Why are all my emails sent via API failing? | Zoho ZeptoMail

Why are my emails sent via API failing?

There are several reasons for your email sending via API to fail. Below are some reasons for it:
  1. One or more mandatory Fields - "subject", "from", "to", "cc", "bcc", "reply_to" - were set as Empty Value or Invalid Value.
  2. The application's JSON requests do not follow a proper syntax. 
  3. The API endpoint URL is incorrect or invalid.
  4. Email-sending domain is not verified. 
  5. There is a mismatch in the email-sending domain and Agent used. The email-sending domain should be associated with the Agent you are using. 
  6. Your ZeptoMail account is yet to be reviewed by us.
  7. Send mail token is invalid.
  8. The File cache key is invalid.
  9. Fields exceeding limits:
    1. reply_to < = 40
    2. to < = 340
    3. cc < = 100
    4. bcc < = 100
    5. Total number of attachments exceeding the limit (<=60 attachments)
    6. Character limit (<=250) exceeded for the name field for any of these parameters - from, to, reply_to, cc, bcc.
    7. Subject field exceeding the character count limit (<=500).
    8. Exceeding the character limit in attachment filename (<=150).
    9. Attachment size exceeding the limit (<=15 MB).
Refer to our help guide here that lists the error codes and solution for each error.

If you are still facing difficulty in troubleshooting, write to support@zeptomail.com with a screenshot of the error.