I just thought I would say something that nobody has said yet, or I haven't seen it said yet so I will say it here, sorry if I get a bit technical.
You can have all the software setup to create bulk emails using software like Mail Merge which gets a set of information from a database / spreadsheet and applies them to an email template, tbh I have never used this myself, but I have seen people using them.
But if that mail server sending all these emails at the same time are not setup with all the required software required these days to send bulk emails may / will get blocked and sending email address / mail server will get blacklisted.
So the sending mail server will need the following:
DMARC (Domain-based Message Authentication, Reporting & Conformance)
SPF (Sender Policy Framework)
DKIM (DomainKeys Identified Mail)
SRS (Sender Rewriting Scheme)
If these are not installed on the server sending the emails out you run the risk of getting blocked and then blacklisted.
We found out the hard way and it is a lot easier to get black listed than it is to get unlisted.
As far as I am aware going by the emails received from BT mail Servers like BT Staff and Openreach and Wholesale, they didn't have the above software installed, so they could get blocked, but the last email I have got from the BT Mail Servers was the end of 2017, so they might of updated their mail service.
I can say Gmail and the mail services owned and run by Microsoft use them plus their own stuff on top.
Paul