Why is it that during a send out, the delivery for addresses of some ISPs is slower than all other addresses.
These providers are known for having a tough policy on email delivery from massive senders. So the delivery on those addresses will be slower because they will be spread over time.
Email takes a long time to reach my inbox, why?
You probably have a lot of emails delayed by ISPs. Some ISPs are known to be picky on emails threshold that you can send per minutes.
This happens when you send to too many wrong email addresses during small periods of time. The ISP will automatically block the following or slow down the reception.
The counterpart is when you have a big queue of emails, the MTA (Message Transfer Agent server) has to manage both sending and receiving queues and it affects the rest of the blast because too many connections are assigned to manage and retry the accumulating queue of delayed emails.
That's why we strongly recommend to never buy a mailing list, much less use one.