Your script gets the message via standard input (STDIN). Try:
#!/bin/bash
echo "Trying to get e-mail" > success.txt
# append data read from STDIN to success.txt file
cat >> success.txt
BTW for more complicated scripts use custom lock to avoid running two scripts in parallel:
:0 w :fooscript.lock
* ^From:(.*\<)?(test@gmail\.com)\>
| /var/log/user1/fooscript.sh