[exim] VERP questions: deliverability, method and execution

Hello All,

Currently our company setup uses Exim version 4.77 #2 built running on a
VPS using WHM to which we have terminal and root access. We have a Mail
User Agent (phplist) capable of creating VERP error-to headers.

We have several questions and I hope they can be answered: I've googled
them but they are either too technical for our fledgling company or
leave out information necessary for a complete understanding.

First, do the users of VERP experience greater deliverable rates
compared to those senders who forgo VERP? It would be great to know if
implementing it in addition to other best-practices helps in deliverable
rates. (In our setup the Mail User Agent can only create VERP headers
when using phpmailer and not SMTP, and therefore an unsubscribe link in
the headers cannot be created, perhaps adding to lower deliverablity rates?)

Secondly, would we be better of getting Exim to produce the VERP
error-to headers rather than our Mail User Agent -- would doing so cut
down on overhead and processing memory?

I do have a third question but that depends on the answers to these two
so I will wait and see what comes back.

Thank you all kindly.