Gmail doesn’t support SMTP by default any more, you have to explicitly opt into legacy / insecure application access or whatever they call it. If that’s still allowed.
With SendInBlue I don’t know how else to interpret the error other than you’ve specified the wrong username and or password in your email profile. Maybe the username needs the “@whatever” suffix or maybe it doesn’t?
Or 530 was hardcoded to "Invalid username or password" somewhere along the line
@Kevin.Herron hit the nail on the head. Allow less secure apps and try again--you'll have to do this anyway. I've personally dealt with this same thing and in my case "invalid username or password" was a red herring
I think that gmail also sends an email that you have to click a link on or something when you initially try to send the email from Ignition to allow that app to send email. going off memory so I may be wrong, but def check the inbox.
I did have to open a support ticket to activate transactional emails at Sendinblue. For my issue, they had responded the same day. I think it also helped that I put in links to Redmine and Inductive Automation to give them a complete picture of what we do.
In 8.1.24+ there is support for OAuth clients and an OAuth email/SMTP profile, which is what should be preferred when doing email with Google, Office365, etc...