Emails sending problems

My test email are working , but when we want to send an invoice the mail send, but the receiver does not receive the email. It has worked for a long time but since this weekend the problem has started.
Any information how to fix this problem

is this the case for all your clients or just one client?
please check if your client email id in Manager was not wrongly updated.
also ask your client to check if the mails are going to their Spam or Junk folder. if this is the case ask them to mark it as Not Spam.

No this happen to all my clients