Can send with zoho account in Gmail but won't receive it in Gmail.

Can send with zoho account in Gmail but won't receive it in Gmail.

Hello.
First of all: Thank you for your great work!!
Some days ago I bought a domain on 1&1. After I have set up some email accounts on ZOHO. After I followed the instructions to include my ZOHO email account in GMAIL. Everything worked great. I tested several times sending and receiving without any issue.
Today I thought to make another test. I have sent one email from my zoho account included in Gmail to the same account.
I waited some time but I didn't receive it in GMAIL. I checked sent mails an it was sent. I went to zoho.com/mail and saw that the sent email has arrived there.
After I sent an email from a standard gmail account to my zoho mail. I saw again that it was sent in Gmail, I didn't recieve it in Gmail but did so in zoho.com(mail. What can be wrong?

Greets,
Sven from Spain

btw: my zoho email account: sven@alipromo.es