You may find this comparison of MTAs interesting. Yes it is dated but Exim was slow as a dog back then. Here is another comparison which is a bit less dated and still shows Exim at the back of the field.

Now the hardware used was relatively old and slow but this is not a hardware bottleneck task. It is concurrency, network connectivity, and DNS lookups that are bottlenecks. To send an email your server has to look up the address in DNS (you want a big local cache), then make the connection to the remote server (may take seconds), then send HELO,MAIL,RCPT,DATA,EOM - all of which can be slowed down by the remote server. The entire transaction can easily exceed several seconds, thus you require multiple concurrent processes (50+) in your MTA to get any sort of decent throughput.

You will note that performace in terms of emails per second is a feeble 5-15 depending on MTA. At 20/second througput you are looking at 20 hours runtime for 1.5 million emails.

I suggest you look at a dedicated mail server that also runs its own DNS with a huge cache to do this task. Exim looks like one of the less efficient options judging from the performance benchmarks. If you dump 1.5 million emails onto your Exim queue and it is really only running a throughput of 5/sec you will effectively freeze outgoing email for 80 hours as any new message will go to the back of the queue. Even if it is doing 50/sec there will be nothing else going out for 8 hours - unless of course there is some way to flag it as low priority. Not only will you have an outgoing email problem you will also have an incoming email issue as Exim is also recieving incoming connections.

If you did want to split up the flatfile of email addresses you don't need a database. Just use split(1).