My companys E-Mails are identified as spam (Mailserver Configuration Error??)

I have a problem with my Domain tackleking.de. We use google for mail transport (mx records to google server, and login on google page) For Mailserver i use Google, but most of my customers tell me that my emails are found in the spam folder. Before we switched to google this was never a problem. so i guess i maybe have any configuration error or whatever. our email accounts are used for business, so this is a big problem when many customers do not receive our emails, becouse they do not watch their spam folder. please make a fast scan on my configuration and if you find any errors or if you really think you can solve my problem then make a bid!