Problem with checkmx

[HOMA S. <hom...@gm...>]

Hi all
I Installed the checkmx  as it was said and put it under the [mail] in
smtpplugins.it works fine but unfortunately  it rejects the email from
gmail and yahoo.
the error is:
Connected to x.y.w..z but sender was rejected.
Remote host said: 553 sorry, 550 No SPF/MX record for sender's domain! (#5.7.1)
is there any way to check the validity of sender domain without such an error?