Beginning last night (12-21-10) between approximately 10p and 2a MT, AOL managed to lose their MX records which are used to direct inbound email such as email sent from SendGrid destined for AOL.  During this four hour period, any email sent to AOL would have hard bounced, causing an entry in your bounce list. Further attempts to send to an email address in your bounce list will be automatically dropped as a means to protect your sending reputation.

If you are having difficulty sending mail to one or more AOL addresses, please check your bounce table for the email addresses in question. You will need to delete any entries in order to have SendGrid attempt delivery for future emails. We do not automatically store or retry any dropped messages affected by this AOL outage.

More information can be found on the Return Path Blog