I'm beginning to think that my emails are being identified as spam.
I'm particularly worried about emails sent via a PHP script through my
domain. On my site people subscribe to my newsletters by typing their
email into a web form. When they do, the system sends out an email
with a link they click on to confirm their desire. But my confirmation
rate is terrible?maybe 20%?and a few people I've contacted said they
wanted to sign up, but never received the email.
Another sign: I see people signing up again and again, but never confirming.

I'm wondering if my emails are being identified as spam. I've gone
through the process on some emailboxes I have?Yahoo, GMail?and it
worked. But that doesn't seem conclusive.

So, how do I find out if my emails are getting blocked? And how do I
prevent this from happening, especially without changing my hosting and scripting?

Other notes: (1) I am hosted by a reputable company, OLM.net, getting
a medium-range offer. I have a "virtual private server," with IP-based
hosting. (2) My company is totally on the up-and-up. I don't give the
emails to anyone for any reason. I never send spam. In fact, I don't
even send newsletters out very often, but I'd like to start doing it
and wonder at my low confirmation rate. (3) I cannot switch to a
commercial alert system. I have too many different newsletters, and I
don't want to deal with Yahoo or etc.

Can`t help you with the technical jazz but how are your emails showing
up on their computer. Sometimes I get them , and they show as Jeff or
some other name. I just delete them to avoid virus`. Try putting more
detail in the I.D. [?]line so recipients clearly know who they are
from. I have probably cancelled emails from friends who were not on my
safe list. Cheers.

I can think of a couple of scenarios that would see your
communications being identified as spam:

1) Your ISP has been blacklisted and you are collateral damage. I
found this at epinions in a review of olm.net: "...the administrator
for another mailing list happened to notice one of my emails had been
flagged as being sent from a relay server listed in the SPEWS
anti-spam database." Once a server has been blacklisted it can be
difficult to get it off the list. You can check multiple blacklists
for your or your ISP's domain here: http://moensted.dk/spam/

2) Some spam filters will falsely identify mail as spam if it has an
arbitrarily "large" number of addresses in the To:, CC: or BC: fields,
as would be the case for a newsletter or other automatically generated
message. If this is the case, you could try sending individual
messages rather than mass messages, although this would probably put
more demand on your server (and might attract the attention of your
ISP).

3) Although unlikely, your ISP might be blocking your email thinking
that it is outgoing spam because of the volume/address situation
mentioned above. However, they would probably notify you if they
suspected a problem.

To really identify the problem could involve a fair bit of
investigation and the cooperation of your ISP and the ISPs of people
not receiving the mail.