[exim] Domain-IP correspondences

My mail server is configured with 2 IP's and virtual domains (xxx.xxx.xxx.180 and
xxx.xxx.xxx.181). I have addressed mydomain.com in DNS to xxx.xxx.xxx.181 (MX records too).

Exim is listening from all interfaces, but when mail is received from mail clients on
xxx.xxx.xxx.181 mydomain.com, it is sent from xxx.xxx.xxx.180. This conflicts with mydomain.com
SPF records (I want to avoid possible domain spammers on xxx.xxx.xxx.180).

Can exim send mail from where is listen IP? Where to tell exim domain-IP correspondences?

Tnx in advance :-)