Email sent to Office 365 users always marked as Junk?

Hi Guys Currently, have a problem with our outgoing email, it does not matter which customers domain it comes from and still happens if sent via IMAP on an email client, or using Roundcube webmail, it always ends up in the Junk side if it's sent to an Office 365 user. We use SMTP2GO as a smart host for sending out all outgoing emails, and this has kept deliverability to a really high standard but does not seem to be working any longer. I also tried to remove the smart host but it made no difference?? I see in the email headers the emails always have "Received From: our ip address & mail server" as a hop before the smart host, is there some way to remove this if for some reason they've marked our server as bad? So only the smart host shows up? The thing that is frustrating is there seems to be no way to diagnose why this is happening, we have valid SPF, DKIM, users aren't sending out mass mailers, we use a smart host (but still happens when off) mail server appears to be configured correctly (nothings changed and suddenly this issue after a year of use) but our "SCL Level" (spam confidence level from Microsoft in the email header) is always 5 for any of our customers domains!!!! Any ideas or suggestions welcome