I'm not with Support, so I can't tell you for sure what happened, but there are some things to check. Was this all outbound mail from your LP account, or only for some customers? If some, anything in common, such as the same recipient email system? Maybe it's blacklisting your LP server because it thinks it's spam, or someone reported you as a spammer (but normally you should get a bounce notice). Do you know if this involved only one application (e.g., a shopping cart), or was it anything sent from your LP account? If just one application, maybe it malfunctioned. Were these individual emails, or bulk sends (e.g., a newsletter)? Are you sure that you kept under your LP email rate caps (so many per second, per minute, per hour, whatever)? Those caps include incoming emails too, so if you were receiving a lot of emails at the same time, your outbound mail might have been dropped. Did you just change anything, that might have erased a cap in your sending application? Did email come back by itself, or did someone quietly change an incorrect setting (such as after a site update)? Is there any logging of outbound emails from your application, or error logging, that might have told you what was going on? Don't forget to look around for new/updated error log messages on your site files, such as error_log files from PHP.