When using Outlook Web App to manage Block or Allow Safe Senders in Exchange Control Panel (ECP), when you check (enable) the “Trust e-mail from my contacts” option and click Save you receive the following error:
Junk e-mail validation error. Value: <E-mail Address>
There are more than 1,024 e-mail addresses in the Contacts folder. This exceeds the default Safe Sender list limit.
To resolve this issue, increase the default Safe Sender list limit of the mailbox. Use the Set-Mailbox command together with the MaxSafeSenders parameter in the Exchange Management Shell.