I have looked at the partial configuration posted and do not see any particular problems with it.
Anton's advice about changing the static route would be very appropriate if the interface used in the static route were Ethernet. But the static route in the original post is quite fine to use the outbound interface, since the outbound interface is a point to point tunnel.
The ping test results seem to demonstrate that the tunnel is up and is passing traffic (at least traffic for a specific destination from a source that is directly connected on the second site router.
The original post says that there are problems with access to the mail server. But it does not tell us what is the address of the mail server or what was the source address that was trying the access.
The original post does show a static route configured on the second site router. But it does not tell us anything about what routes are configured on the primary site.
I am guessing that the problem is a routing issue. But we do not have enough information to identify the problem. If we had information about what routes exist on the primary site and had information about the source and destination addresses were having problems then perhaps we could suggest answers for this problem.
We are pleased to announce availability of Beta software for 16.6.3. 16.6.3 will be the second rebuild on the 16.6 release train targeted towards Catalyst 9500/9400/9300/3850/3650 switching platforms. We are looking for early feedback from custome...