Manual order status change fail to send mail

[KimCK]

When we get an order a mail is send to the customer, all fine. 

Then when we change the status of the order, e.g. prepare or cancel the order a mail should go to the customer but fails. A resend of the mail fails too. 

We have had this as a ongoing issue and retrying 3-10 times made it work. However now it fails 100% and we cannot send any mails when changing the status of the order. 

The error is this one "Swift Error: Expected response code 250 but got code \"\", with message \"\""

Remember the first 2 mails works just fine -> order confirmation and payment.