Changed the all modified field on the email after sending another email

Hi Experts

I observe that after sending another email using a custom workflow it changed also the modified of all emails. Do you have any idea about this?

Here's my code to send email 

SendEmailRequest sendemail= new SendEmailRequest
{
EmailId = emailID,
TrackingToken = "",
IssueSend = true
};
SendEmailResponse response = (SendEmailResponse)service.Execute(sendemail);