The Menlo Park company said the first issue, that of undeliverable emails, was happening because users only receive emails to their @facebook.com inbox if they are sent from email addresses listed by either their friends or friends of friends. For all other cases, those emails would go into an "Other" box, of which many users are unaware.

There are also some users who adjusted their settings to not accept any emails from people other than their friends and friends of friends, and in that scenario, Facebook is supposed to notify the sender that their email bounced back.

But in some case, the company said, bounce-back emails were not being delivered, leaving senders and intended receivers without a clue that the email had failed. This was a result of spam filters intercepting the bounce-back emails. Facebook said it is working to resolve this issue.

As for synced address books that were affected by the change, Facebook is notching that issue up to a bug.

Facebook said synced address books work with an API that is supposed to draw users' primary addresses, and the company said from what it can tell, that API is working with most devices correctly for the most part. However, some devices were drawing the last email added rather than the primary email, which caused the problem. Facebook said it is also going to fix that issue soon.