Words in russian language have parts.
Shortly(very shortly, russian suffix is english derivational suffix, russian ending is english inflectional suffix):
prefix-root-suffix/ending/postfix
Example: unreachable
un - prefix
reach - root
able - postfix
послать
по - prefix
сла - root
ть - postfix
ergo, words "слать" and "послать" have one root
слать too close "send"
послать too close "send to"