Support topic: http://www.phpbb.com/community/viewtopic.php?f=46&t=1996735

Code: http://tps.projects.bsdpower.com/browser/branches/phpBB-3_0_0/phpBB/includes/functions_messenger.php#L674

When hosts impose hard limits on outgoing email amount, ignoring package size when it is too small and sending huge quantities of email results in mail being dropped. Board administrator is unable to do anything to get mail queued and eventually delivered.

Boards can be reasonably expected to have spikes in mail usage. Mass mailings are one example. When such spikes occur it is ok to hold at least some mail for extended periods of time. E.g. if I'm mass mailing 1000 or 5000 users and some of them receive my message 2 hours after I send it that's really fine.

Perhaps a priority can be assigned to emails so that mass mailings have lower priority than other email. This way an admin sending 1000 emails does not compromise timely delivery of topic, etc. notifications that are much fewer in quantity while the 1000 emails are being processed.