I see quite a few questions about emails not being sent, I think what I'm experiencing is somewhat different than the majority of the problems out there.
Typically my transactional emails (orders, invoices, shipments) are successfully sent but some % of them never get sent. I've noticed that with both automatically generated emails and also manually 'resent' emails from admin interface. Sometimes I have to manually resend multiple times.
My practice is to BCC all outgoing emails to an email account so I have a record of exactly what emails are sent. But a good % of transactional emails are never received by customer or my BCC account.
I don't see any relevant errors in logs, Magento or PHP. Any suggestions for tracking this down?