Ways to reverse simple encodings

I'm doing a blackbox website pentest for a client. I found a way to the database but the client won't accept something he doesn't understand. He's requesting users credentials.

In the database the password is plain text but the email is encoded (and not encrypted, i'm sure of that). Here's an example: xsq%sw7p9ipq3z06qs7awdiq(< email >)

Note that "< email >" is static.

I would like to know if any of you have a method to try to decode this and maybe any simple encoding.

Regards, Hamza.