Why are my emails sent via SMTP failing?

There are several reasons for your email sending via SMTP to fail. Find below the list of possible reasons:

trying to connect to SMTP server in un-secure mode.
Invalid Send Mail Token or Username.
Sending email from an invalid 'from' address domain.
Sending email with an empty 'Subject' field.
Detection of virus in eml file.
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.

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