With the amount of spam we are getting today, whitelisting and blacklisting of emails addresses or domains is a norm. Whitelisting / Blacklisting is when you either want to allow / deny an email / domain from sending you an email. Most mail filters can do this at a domain level. There are times which users would like to do this at a user level. This can done in Outlook Web Access. Another case is when you have multiples domains on an Exchange server and wish to allow or block someone. This has to be done on Outlook Web Access. There is a limit on Exchange which is 1024 MB. Yes, every time you add a user or domain, it is taking up space. When you reach the limit, you may get this error:
This exceeds the default Safe Sender list limit.
In order to resolve this, you will need to log on to the Exchange Management Shell or Azure Powershell. The attribute to amend is called the MaxSafeSenders limit.
Run the following command: set-mailbox User@domain.com -MaxSafeSenders 2048
This should be an instant change on the system and you do not have to wait for replication.