Hi, 180,000 emails per hour means 50 email per SECOND, I really doubt even a good server can support that load. More the PHP may have an execution max time that can be raised by your system administrator, for example to 3 minutes (may be now is a lower value, you can find it on the diagnostic panel).
To send such volume per hour you need a cluster of sending processes, it’s not something can be (easily) implemented with a blog.
Try to rise the PHP max execution time.