HTC Desire S :: Can Receive Emails But Not Send Or Reply?

I can receive emails on my unlocked HTC Desire S and can on my original unlocked HTC Desire but cannot reply or send fresh email on either. The settings are OK as they worked previously, in fact only minutes earlier on the Desire S.When satisfied it all worked OK I took the Vodafone SIM out of the Desire S and put it in the Desire to see if the email issue still existed. It did. I put the SIM back in the Desire S and the problem has replicated itself. Now all I can do is receiveI am reluctant to put the SIM in my unlocked Samsung Galaxy S2 in case the problem occurs again.