[kh99]

On the admincp home, in the box with the stats, do you see "Number of Queued Emails" (it should be at the bottom of the right column)? It could be that your emails are queued and being sent gradually, as you guessed. If you have queued emails, you can control how fast they're being sent by setting "Number of Emails to Send Per Batch" in the Email Options. If "Use Cron Based Sending" is set to yes, they are sent every 10 minutes, so you can control how many you're sending per hour (approximately) by setting "Number of Emails to Send Per Batch" to 1/6 that amount.

The "Use Cron Based Sending" was added at some point (maybe in 4.2.2, I don't remember), so that's probably why you started having the issue after the upgrade.