I have just availed your full email service . I added mx records for my domain as :-
1:
MX
@
roboconn.ltd
mx2.dynu.com
1
10 minute(s)

2:
MX
@
roboconn.ltd
mx1.dynu.com
1
10 minute(s)

3:
TXT
@
roboconn.ltd
v=spf1 mx a:mx1.dynu.com a:mx2.dynu.com -all

Now I can send and recive email for the mail(abdussalammahmood@roboconn.ltd) i added when in dynu webmail.dynu.com.
But then i added email to iphone email app from my phone using iPhone email app i recieve emails fine but when i send them app says "The sender address 'abdussalammahmood@roboconn.ltd' was rejected by server"