When adding recipients to a campaign, if you have an excessively large database we will disable the option to "Send to ALL Contacts".
Sending to a large number of contacts at once can trigger spam filters, resulting in your emails not even reaching your contacts inbox. Repeatedly sending to a large database can also harm your contact relationships, as well as your sending reputation, increasingly reducing your deliverability.
When Send to ALL Contacts is disabled for your protection, we suggest that you choose refined groups of contacts, Tags or Audiences that your content most relates to, perhaps only targeting contacts that have shown interest in similar content that you have previously sent, using campaign Conditions.
You can also consider enabling SmartSend, or sending your email in smaller recipient groups, in spaced out campaigns. Regularly cleaning up your database will also improve the overall health of your emails.