Microsoft Outlook outgoing emails diverted to recipients spam or junk for hotmail outlook recipients

Hello

We use Outlook as part of Office 365 in our business and recently we have seen that our outgoing emails to our staff and clients with hotmail or outlook email accounts are being diverted to their spam / junk folder.  The recipients have tried moving these emails to inbox, adding us as a contact in their email account and also as a VIP but nothing changes.  All other emails to gmail, yahoo, Mac etc are received into the inbox without any issues so this is an issue related to Outlook client to hotmail and outlook.com addresses only. The email server is a GMAIL server and if we logon to the account via the browser, the emails are received without issues.

We have had no issues for 3 years and then suddenly we see this issue which is annoying and time consuming to get around.

There are requests for help on this platform and elsewhere about this behaviour but seemingly no solution although there is an article which mentions changing DNS entries at the domain provider level.

I tried to seek help via MS support for Outlook but they suggested I ask all our clients and staff with these type of email accounts to review their setting in hotmail or Outlook.  Clearly this is impossible as most of our clients are elderly and not tech savvy in any way.

Does anyone have any ideas on how this can be resolved please?  Surely this is a Microsoft issue?