Sendmail - Centos

I am receiving response like below when sending voicemail to email in centos
This is a multi-part message in MIME format.

------voicemail_81024888610744638
Content-Type: text/plain; charset=ISO-8859-1
Content-Transfer-Encoding: 8bit

Dear 102 ,
You have received a new voicemail from “xxx”
on Thursday, February 14, 2019 at 10:41:48 AM.
Find attached.

Regards>>>
xxx

------voicemail_81024888610744638
Content-Type: audio/x-WAV; name=“msg0007.WAV”
Content-Transfer-Encoding: base64
Content-Description: Voicemail sound attachment.
Content-Disposition: attachment; filename=“msg0007.WAV”

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It has been generated in var/spool/mail/root file

Looks OK to me, although you are missing the RFC822 headers from the message as a whole. What mail client are you using?

I am currently using outlook.com for sending the mail

what do you mean by RFC822 headers?

RFC 822 is the standard for internet emails. The headers are the the part of the mail that provides the meta data. One of those headers is a Content-Type header, which would tell a receiving mail program that was even vaguely modern, how to interpret the body as a message and attachment. It appears that your mail system is behaving as though that header is not there, but you have not provided the headers, so we can’t see whether it is really missing.

Those headers would be present in /var/spool/mail/root. Even if they were missing in the incoming mail, the mail handling system would have created at least some of them.

I have no knowledge of outllook.com.