Q:

How to set from address in Jenkins with email-ext plugin? 

Using AWS SES as mail server.  Set smtp, username and password at Extended E-mail Notification area.  But didn't  find where to set from address.  If don't set it, the send mail will be failure:
SendFailedException message: 501 Invalid MAIL FROM address provided

At Jenkins Location area, there is a System Admin e-mail address input.  Even set an address there, can't send mail successfully. 

A:

We use Jenkins pipeline and we set the from via the emailext option:
def subject = "JENKINS-NOTIFICATION: ${buildStatus}: Job '${env. JOB_NAME} [${env. BUILD_NUMBER}]'" 

emailext(
            mimeType: 'text/html',
            replyTo: '$DEFAULT_REPLYTO',
            subject: subject,
            from: 'jenkins@host.com',
            to: 'dummy@corp.com',
            body: '${SCRIPT,template="email.template"}',
            attachLog: true,
            compressLog: true,
            recipientProviders: [[$class: 'DevelopersRecipientProvider']]
    )

See also the documentation on which options you can define.