Debug slow sending

I am sending out a newsletter to a huge list, some email addresses dating back as far as 2002 when I started collecting them.

I set the batch size to zero and the throttle to 1 email per second.

At first, the first 2 days, emails where flying out by the tens of thousands and everything worked well.
Now, for the last x thousand emails, progress has been very slow and it has been several days now.

I’m sure there is a valid reason for that. I just would like to understand it. Is it because these email addresses are on domains that don’t exist anymore or something like that ? Is the email command synchronous and is it because every email now takes a lot of time somehow ? What other reasons could there be ? Is there a way to debug/see the reason ?


Perhaps this is the solution for your problem:

Since I installed that files, phpList sends as fast as before.

Thanks @arada but I don’t think this is related no.
There is no visual aspect to the ‘problem’ I’m reporting. This is just based on stats when I go check after a night sending. There is no page opened during sending.