emails from gmail bounced

[sistemas@nnttr.com]

Hi,
we have Zimbra Network Edition 8.7 with this configuration:
- master domain DOMAIN1.COM where all users are created
- secondary domains for each company in the enterprise: DOMAIN2.COM, DOMAIN3.COM, etc.
the principal domain for all users is DOMAIN1.COM, so the email adress is user1@DOMAIN1.COM, but every user has aliases like user1@DOMAIN2.COM, user1@DOMIAN3.com...

CASE1: when user1 sends email with user1@DOMAIN1.COM, envelope.from is user1@DOMAIN1.COM, header.from is user1@DOMAIN1.COM
CASE2: when user1 sends email with user1@DOMAIN2.COM, envelope.from is user1@DOMAIN1.COM, header.from is user1@DOMAIN2.COM

we don´t have any problems with this, and works fine. In CASE1 we get alignment in SPF and DKIM, in CASE2 we get alignment in DKIM only, but this is OK for DMARC

If we send emails to GMAIL there is no problem also, unless we send emails to 5 o more gmail recipients.

When we send email to more than 5 gmail recipients, in CASE1 emails are received, but in CASE2 emails are bounced with this error:
4.7.0 (other or undefined security status)
smtp;421 4.7.0 [77.226.209.120 15] Our system has detected an unusual rate of unsolicited mail originating from your IP address. To protect our users from spam, mail sent from your IP address has been temporarily rate limited. Please visit https://support.google.com/mail/?p=Unso ... LimitError to review our Bulk Email Senders Guidelines. f18si1649238wro.92 - gsmtp

We have checked https://support.google.com/mail/answer/ ... 23471&rd=1

Why is gmail processing my emails as BULK when the envelope.from is different from header.from?

Any help would be very appreciated. Best regards,
JJ.-