My emails are not sending to multiple contacts sometimes but going to my outbox and then I still can't send from there. Usually with very small attachements .
Does the same thing happen if you use only the webmail interface, and not the client/app? How are you accessing the email?
checking the server logs, a device is failing the password check when sending emails. You will want to identify the device and correct the outbound password setting to resolve any issues in sending.