Replying to forwarded messages are marked as spam by recipient

Replying to forwarded messages are marked as spam by recipient

Copy of  https://productforums.google.com/forum/#!topic/gmail/flzJzxnWoFQ;context-place=forum/gmail

I have 3 accounts.

1@gmail -> gmail
2@zoho -> zoho
3@gmail -> gmail

2@zoho forwards all his emails to 1@gmail, 1@gmail answer both his emails and mail from 2@zoho by selecting the "From" field.
I set the forwarding from 2@zoho, which delivered a configuration email on 1@gmail and then entered the verifying code back on 2@zoho.
If I send a message directly from 1@gmail -> 2@zoho, 1@gmail -> 3@gmail, 2@zoho -> 1@gmail, 2@zoho -> 3@gmail, 3@gmail ->1@gmail, 3@gmail -> 2@zoho all emails are encrypted and  not marked as spam.
Plus this is my zoho setup, everything is in order:



The problem is when I send a message from 3@gmail to 2@zoho, which will be forwarded to 1@gmail. Then on 1@gmail I will reply using as "From" the identity of 2@zoho to answer 3@gmail. Now 3@gmail sees the message as  not encrypted and  marked as spam.
So why are my mails flagged as spam?
I don't know if the problem is with zoho or gmail or whatever…