iRule Recommendation

A simple redirection irule should do the redirection :

when HTTP_REQUEST { 
  if { [HTTP::host] eq "webmail.abc.com" } { 
    HTTP::redirect "https://webmail.xyz.com[HTTP::uri]"
  } 
} 

 Then you old irule should take care of pool selection :

when HTTP_REQUEST {
  if { [HTTP::uri] starts_with "/owa" } {
    pool OWA_pool
  } 
  elseif { [HTTP::uri] starts_with "/example" } {
    pool Example_pool
  }
  #...
}

If irules are associated to the same VS, yous should make sure the redirect execute first, you have a number of options for that including checking for the hostname before pool selection, or adding priorities to your events