The scenario is that bounces weren't being fetched and there is now a major backlog, and that running 50 every cron is too slow to make real progress.
1 Answer
Here is one approach - modify /civicrm/CRM/Utils/Mail/EmailProcessor.php
so it has multiple iterations specified (in example below = 40) then run via
drush cvapi Job.fetch_bounces
around line 178
// process fifty at a time, CRM-4002
$iterations = 0;
while ($iterations < 40 && $mails = $store->fetchNext(MAIL_BATCH_SIZE)) {
$iterations++;
print("\nBeginning iteration $iterations ");
foreach ($mails as $key => $mail) {
print(".");