Creating Custom Senders makes it easier to view reports about messages sent from the CIDRs associated and to Approve that custom sender for a particular domain.
To view the custom sender, goto Configure > Manage Custom Senders or Diagnostics > Senders, The Source/Source Type column tells you the source of the associated CIDRs; if they were added Manually or Discovered via a DNS SPF or MX record.
Sometimes, all or some of the CIDRs associated with a Custom Sender will be removed automatically by the system. This can happen when an SPF record, for a verified domain, contains a DNS entry (SPF or MX) with a CIDR that subsumes all or some of the CIDR(s) associated with the Custom Sender. CIDRs cannot be associated with more than one sender. If Brand Protection detects a CIDR that is larger than the one manually associated with a Custom Sender, we will move the larger CIDR into the Unassigned sender and leave the Custom Sender without those CIDRs associated. The process that checks for CIDR overlap runs once a day around UTC midnight.