"The value cannot be longer than 1024 characters."

This character limit is by design.
Workaround: use theĀ InternalSMTPDomains Registry setting instead.
Note: Adding domains to the InternalSMTPDomains Registry setting will not automatically add them to the "List of internal SMTP domains" site setting, and vice versa. The lists are independent, and Enterprise Vault's agent simply uses the union of both lists to determine which domains are internal.