I found the solution for anybody who come accross such problem (550, "5.7.1", Invalid credentials for relay. for Google GSuite)1. Log into the GSuite admin console and head to the email report (https://support.google.com/a/answer/2604578) in order to check email logs2. Message detailsSender:firstname.lastname@example.orgMessage ID:<5908540c.ec9adf0a.23d7d.c6d3SMTPIN_ADDED_REJECT_SESSION_5395003@example.com>Date:May 2, 2017, 11Error details:The connecting IP address 91.XXX.248.235 might not have been included in the list of allowed IP addresses in your SMTP relay settings. Otherwise, if TLS encrytion was enabled, there may have been an issue negotiating the encrypted communication.For more details, visit SMTP relay service setting.3. Log in back to GSuite admin console > SMTP relay, add the IP above (91.XXX.248.235) > working!Now why OVH displays that my server IP is on 87.XX.235.184 (and it is, I have a A record on this!)and my SMTP see email requests from 91.XXX.248.235?No idea. If anybody can jump in with explanations? My guess is that this is a way for OVH to optimise their infrastructure? maybe...Anyway, problem solved. Thanks.