Looks like the user doesn't exist on that system.
You still are not following directions either.
You can't login using "user1@ip_address1"
echo "$passvar" | ssh -p 44 $i "sudo su - 'passwd --stdin $uservar'"
And have a valid user name in $uservar.
Also use code blocks to prevent misformatting.
One last thing.. if the file ip.txt has "ip_address1" it won't work unless that is the host name for the server.
And I did make a mistake here. Sorry: I left out the -c option to su, that line should be:
echo "$passvar" | ssh -p 44 $i "sudo su - -c 'passwd --stdin $uservar'"