Linux – How to Specify IP Address for Outbound Connections on Multi-IP Host

one of my servers (Debian 5.0.6) has two public ip-address on the same interface. This used to work well for months but suddenly it is using "the wrong" ip addresses for outgoing connections. This is a problem because the reverse lookup will not match and emails therefore get spam points.

eth0      Link encap:Ethernet  Hardware Adresse 00:1b:21:14:8e:9c  
          inet Adresse:81.169.180.51  Bcast:81.169.180.51  Maske:255.255.255.255
          inet6-Adresse: fe80::21b:21ff:fe14:8e9c/64 Gültigkeitsbereich:Verbindung

eth0:0    Link encap:Ethernet  Hardware Adresse 00:1b:21:14:8e:9c  
          inet Adresse:85.214.157.120  Bcast:85.214.157.120  Maske:255.255.255.255


Kernel-IP-Routentabelle
Destination     Router          Genmask         Flags Metric Ref    Use Iface
81.169.180.1    0.0.0.0         255.255.255.255 UH    0      0        0 eth0
0.0.0.0         81.169.180.1    0.0.0.0         UG    0      0        0 eth0

Currently it is using 85.214.157.120 for outbound connections. How do I get it to use 81.169.180.51?

Edit: The netmask of 255.255.255.255 is consistent with both documentation and DHCP response of the hosting company. Calling /etc/init.d/networking restart multiple times will eventually end up with the correct ip-address for outbount connections. But that is obviously not a stable solution. /Edit

Edit 2: To make sure that the host route is not related to my issue, I setup a local test network:

eth0      inet Adresse:192.168.0.2  Bcast:192.168.0.255  Maske:255.255.255.0
eth0:0    inet Adresse:192.168.0.3  Bcast:192.168.0.255  Maske:255.255.255.0

192.168.0.0    0.0.0.0         255.255.255.0   U     0      0        0 eth0
0.0.0.0        192.168.0.1     0.0.0.0         UG    0      0        0 eth0

If anyone has an idea how to make sure that source ip-address 192.168.0.2 is used on outbound tcp connections, I'd be thankful.
/Edit 2