12:00pm Pacific / 8:00pm GMT — We are pleased to announce that we have found a definitive cause for the unusual spike in traffic in the German datacenter last week.  We have also implemented a change to prevent a recurrence of this issue.

The problem was related to the use of IMAP journaling with a customer that was attempting to archive internal messages with extremely large attachments (in one case, over a gigabyte in size).  The messages were so large that the file transfer was longer than the interval between IMAP queries, and the transfer failures and subsequent retries were consuming available network bandwidth on the load balancer responsible for this traffic.

Most mail servers will not allow messages above a certain size, typically between 10MB and 50MB, and indeed our SMTP-based journaling for the archive service has the same 64MB size limit per message as our email security service.  To prevent the scenario described above from creating an operational risk in the future, effective today we are imposing an individual message size limit of 64MB for messages ingested into the archive via the IMAP journaling process, identical to the existing limit for SMTP-based journaling.  (Only a small number of customers use the IMAP-based journaling, and we continue to strongly recommend the default SMTP-based journaling for our archive customers.)

If you have any questions about this change, please do not hesitate to contact our support team.  Thanks for your attention.