SMTP Error (550): Failed to add recipient

Hello, I have a problem with sending and receiving emails with roundcube webmail, I also use the ISPCONFIG control panel, when I try to send emails from roundcube to gmail, I get this message:

SMTP Error (550): Failed to add recipient “xxgmail.com” (5.1.1 : Recipient address rejected: gmailcom).
Also when I send from gmail email does not come to roundcube.

The work must be done on my computer with team viewer, if you don’t agree with the budget, don’t bid, maybe it’s a matter of minutes, because here I think the configuration is incorrect.