Mails from send E-mail smart service are going into recipients junk folder

Normally Appian E-mail comes in Inbox. Unless the user has previously marked any appian mail as spam / junk or the mailbox system might think appian mails as junk. Ask the user to mark these mail as not spam & see if this works Or you can contact mailbox admins not to mark appian mails as junk

Yes thats possible .But we are  sending emails for specific group .All members in that group facing same issue

I've found this can sometimes happen when there's a mismatch between the "from: email address" and the actual sender domain (which will be your appian instance), for instance when you're overriding the "sender" details with something belonging to an individual user.  Unfortunately this behavior is largely controlled by the recipient's email system and hard to control from the Appian side.