Problem:

Mail forwarding to gmail users not working.


Solution:

According to updated gmail policies for forwarded emails, gmail may reject the forwarded emails.  Gmail requires the forwarded emails having sender authentication which matches the server which forwards the mails. In case of mithi servers, server doesnt change the sender ID while forwarding mail, it forwards the mail with actual sender. 
According to updated gmail policies for forwarded emails, gmail may reject the forwarded emails. Please check the article below.

https://support.google.com/a/thread/239644302/gmail-requires-all-senders-to-authenticate-with-either-spf-or-dkim?hl=en

https://support.google.com/a/answer/81126?sjid=10318237175185842405-AP&visit_id=638467071574778434-857735786&rd=1

We recomend below workaround for forwarding the mails:
Forward the mails to other domains like yahoo, outlook, hotmail etc.
If you are setting forwarding for data backup you can take vaultastic plan.