Why is there sometimes a delay in receiving email?

Email delays can be attributed to the size of the email and its attachments, network latency, or occasionally issues with spam/virus scanners getting backed up. Most often, it's a case of sender or recipient client delays. Many mail clients only communicate with the server to send or receive new mail every few minutes. If network traffic is high, this communication can be further delayed, especially when inbox and outbox sizes are large.