Mailpoet test mail works but newsletters fail with errors 553

Hi there,
I use Zoho as an external SMTP client. I recently changed my account policy and enabled 2-factor authorization. This is the only thing I changed, newsletter sending was working before this change. I’ve generated a one-purpose password for Mailpoet and entered it in the settings. Test mails sent from the settings page work, but newsletters don’t and return the following error:

1 : Error Sending Message Post Pay Counter PRO got updates (v. 1.6.5)! to ******* | DATA not accepted from server 553 Relaying disallowed as Post Pay Counter => Next try in 0 minutes.

Any advice?
Thanks! ??

Have a nice day,
Stefano