SMTP client mail frontend 587 not sending email using send as user.

Hi there,

Thanks for reading my questions below.

I have configured my client frontend to send from IMAP client using port 587, it works on IMAP client. But I have given send as permission to user2 for the mailbox user1, I'm able to send using user1 for the mailbox user1, but unable send email using user2 for the mailbox user1. I would like to know how to send as user2 for the mailbox user1. (or) the send as permission only applicable to exchange connectivity?

Could you please answer my question and How to send using send as for IMAP client?

Thanks for your time.

Kannan