Outbound IP with aliases

Dear experts,

I have a dedicated server with single eth0 interface and four IPs
bound to it using IP aliases.
Poptop is configured to listen to all available IPs.

Even though clients can successfully connect to any server IP address
(including ip-aliased addresses)
all the requests to the target servers appear to be going from the
main server IP address.

I naively expected that if the PPTP client connects to the ip-aliased address
the PPTP server should use the same address for the communication with
the target server.
What should I do to make it work like this? Can this be done with
iptables rules?

My configuration is pretty much standard.

/etc/pptpd.conf contents:
localip 192.168.99.1
remoteip 192.168.99.10-100

/etc/ppp/options.pptpd contents
name pptpd
 refuse-pap
 refuse-chap
 refuse-mschap
require-mschap-v2
require-mppe-128
ms-dns 208.67.222.222
ms-dns 208.67.220.220
debug
dump
lock
nobsdcomp
novj
novjccomp
nologfd

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