e-sign sendDocument service, Invalid email: null error

Am new to eSign API / integration so apologies if this problem is an obvious one.

I have successfully connected using oAuth with scope agreement_send:self and now want to call the sendDocument service. Am using SOAPUi to test. Have stripped tags I don't need (and may have stripped one too many). The error I'm getting is Invalid email: null error.

I assume that the sendInfo section is optional and in absence of it, the token is used to determine who the system is dealingh with.

Any help much appreciated.

My SOAP payload is:

<soapenv:Envelope xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/" xmlns:api="http://api.echosign" xmlns:dto="http://dto.api.echosign" xmlns:dto2="http://dto21.api.echosign" xmlns:dto9="http://dto9.api.echosign" xmlns:dto21="http://dto20.api.echosign" xmlns:dto1="http://dto14.api.echosign" xmlns:dto11="http://dto18.api.echosign" xmlns:dto12="http://dto19.api.echosign">

   <soapenv:Header/>

   <soapenv:Body>

      <api:sendDocument>

         <api:apiKey>MYKEY</api:apiKey>

         <api:senderInfo/>

         <api:documentCreationInfo>

           <dto:daysUntilSigningDeadline>2</dto:daysUntilSigningDeadline>

            <dto:fileInfos>

               <dto:FileInfo>

                  <dto:documentKey>123</dto:documentKey>

                 <dto:fileName>test.pdf</dto:fileName>

                  <dto:mimeType>application/pdf</dto:mimeType>

                 <dto:url>https://MYHOST/MYAPP/docs/test.pdf</dto:url>

               </dto:FileInfo>

            </dto:fileInfos>

          <dto:recipients>

               <!--Zero or more repetitions:-->

               <dto1:RecipientInfo>

                  <!--Optional:-->

                  <dto1:email>MYAMAIL@MYCOMANY.COM</dto1:email>

                  <!--Optional:-->

                  <dto1:fax></dto1:fax>

                  <dto1:role>SIGNER</dto1:role>

                  <!--Optional:-->

              </dto1:RecipientInfo>

            </dto:recipients>

             <!--Optional:-->

            <dto:signatureFlow>SENDER_SIGNATURE_NOT_REQUIRED</dto:signatureFlow>

            <!--Optional:-->

            <dto:signatureType>ESIGN</dto:signatureType>

         </api:documentCreationInfo>

      </api:sendDocument>

   </soapenv:Body>

</soapenv:Envelope>