perhaps the function
will help here?
read the full description and you see that related fields can be called. the SORT of the relationship and/or the portal will determine which value is last.
I don't think that the portal sort order will affect which value is returned by Last. It can, however, create some confusion as the portal's sort order might show one record as "last" while the last function--which will ignore the portal settings and work solely from the relationship's sort order, might return a different value.
One simple way that doesn't require an additional relationship:
Give your portal a name, let your script go to that object, then to the last portal, then execute the Send Mail step.
In a similar vein: let the script Go to Related Record, matching only, of in a new window, go to last record, Send Mail, close window.