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

Why are emails sent via SMTP failing?

There are several reasons email-sending failures using SMTP. They are:
  1. Trying to connect with the SMTP server from an insecure source.
  2. Misaligned port and certification. (TLS—>587; SSL—>465)
  3. Invalid TLS certificate version. ZeptoMail supports TLS v1.2 and above only. 
  4. Your application doesn't support longer password. You can generate shorter passwords from the SMTP/API section of the Agent. 
  5. Invalid Send Mail Token or Username.
  6. The email-sending domain is unverified. 
  7. 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. 
  8. Sending email with an empty 'Subject' field.
  9. A possible virus in the eml file.
  10. Attachment size has exceeded the user policy. Total size of the email including attachments should NOT exceed 15 MB.
Refer to the help guide here to compare your error response code and troubleshoot.

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