Mailserver behind ipsec tunnel

Hi all! I have configured ipsec tunnel between isa and mikrotik. Now i wanna configure email notifications, but its not work. I think its bcs if i set external mailserver address - its the same as used to establish ipsec tunnel and isa refused this connection. If i set internal mailserver address - mikrotik send packets from some ip thats not internal. How can i configure mikrotik to send packets to subnet that behind ipsec tunnel from internal ip, or configure so that packet from (external?) Mikrotik ip will be accepted?
By real example is icmp ping isa and mikrotik each other: on isa i can`t define source ip to ping mikrotik - so ping failed. On mikrotik if i not define source ping ip to isa as internal Mikrotik ip - ping also failed.