phplist doesn’t support cycling through a set of smtp servers nor sending concurrently to more than one server.
Any topics in the old forums are going to be out-of-date now.
It’s not my area of knowledge but I think that these functions are best implemented in the local mail server (postfix, exim, etc).
It shouldn’t be necessary to use more than one mail server.
If you do decide to go this route anyway, I highly suggest documenting every little step you take. A proper implementation of a round robin would require a hack that would be overwritten with the next PHPList update AND/OR server update.
When that happens, you’ll have to set it up all over again.