The problem is, the "reply to" and "from" address do not match when using the multi SMTP mod.
This guarantees failure/filtration on most email clients / spamwares. Been there again and again.
Filters have become pretty strict.
I can (and have) blast(ed) out thousands an hour

Other issues exist like only being able to send ~100 per hour to aol (changes from time to time too) or you are blacklisted but, phplist has auto throttle which addresses that issue to a point.

WE MUST have a way to send thousands an hour and stay within / adapt to receiving domain rules.
I am currently on a dedicated rackmount xeon server with 6 dedicated ips. My hardware is capable of 250,000 emails an hour no problem.

My temp solution was to set up a reseller account on one of my dedicated ips. Install ten domains and ten instances of phplist all running from the same database. I set up the domain throttling (in config.php) based on the domain I am sending to in each install.

Since I set up the individual lists per recipients email address domain. I have a list with just aol addresses, a list with just verizon, then comcast addresses and so on until the few oddballs left over are all dumped into the remaining list.
I can then send from each to specific list and they get to where they are going with hardly any failures if plain text is used.
This takes care of "from" and "reply to" mismatch issues, specific / individual domain throttling issues and allows me to send out at a maximum rate only restricted by the receiving domains anti spam criteria.
BUT THIS IS SUCH A PAIN TO MANAGE and hacking around multi connection issues to get it to play nice with mysql was a hair puller but, after over two years of using phplist and tons of experimental configurations and hacks, this is the only configuration that works for me.
Now, multiple SMTP load balancing would help SOOOOO MUCH and the built in domain throttle could just be set to the worst case scenario (aol) or 1 every two minutes. I can see why multi smtp has not been built in, it is a real pain to do it right due to affecting pretty much every stat and connection routine but, I would pay for phplist if these feats were standard.

@DEVELOPERS - How about a free version and an advanced version with multi SMTP load balancing and SPECIFIC domain throttling rules?