There’s a typo here:
Ignore, I only made that type in the email example.
After some more google research I found another user describing a similar issue so I did some testing:
ds_select_dst does RESET $rU and $tU but not $fU
ds_select_domain does not RESET $rU or $tU.
Doing:
ds_select_dst("2000","6"); $rU = "0012345"; route(RELAY)
setting $rU after calling ds_select_dst solved the issue I have.
But I wonder, why this is resetting certain header. This is not clear from the documentation.
I also wonder: When should one use ds_select_dst and when ds_select_domain ?
Could ds_select_domain be safely used for any situation?