> reverse DNS lookup. It adds for an extra layer of security
> to prevent spam because the spammers might not have access
> to the manipulation of PTR records.
>
spammers can have access to their own DNS and can give PTR. The real security
is that you cannot impersonate/use and open relay/proxy. Any server can claim
that its gnu.org when originating the mail transfer but when the receiver tries
to validate the request with mail.gnu.org, it will be routed to the original
gnu.org server and hence the receiver will know that there is something fishy.
> But I have another doubt, suppose that a particular ISP
> does not allow for changing the PTR records or assigns
> dynamic IPs (like BSNL), then would it not be possible for
> hosting an email server? Would services like Gmail blacklist
> such IPs completely?
>
There is a very high probability that this will happen. It is not mandatory but
if gmail gets suspicious, it will do so (at least 2-3 yrs back). It can get
suspicious based on volume and/or content. Yahoo was much more lenient. Gmail
could have gotten stricter now and blacklist on first try but I wouldn't know
about that.
