URGENT - EMAIL PROBLEMS

Hi. I am having problems with my emails. Emails are appearing twice and three times (these are all the same email) . Emails are being delayed in sending to other Zoho users in the office (12 hour delay) Emails are being delayed when receiving. Someone sent me an email at 10 am in the morning and it was not received that day at all. When I logged in the next morning, it had been received. Please advise asap. thank you