There are a couple of reasons why the email API might not accept your email.
A. Check if you are using the correct email API address for your company.
B. Check if you are sending the emails from the same email account that is specified in your Umapped account.
C. Check if you are sending the emails to the system using the TO: field
D. Check if you are attaching only PDFs and Word (DOC and DOCX) files - all others files will not be supported.