Outgoing mail contol is preventing non-plesk (SSH) users from sending mail

With outgoing mail control switched on my non-plesk users created under SSH receive this error when trying to email out -

Mail handler 'limit-out' said: REPLY:554:5.7.0 Your message could not be sent. The user is not allowed to send email.

With outgoing mail control switched off they can send email just fine.

Any ideas how to work around this?

Thanks