Emailings created from Zoho campaigns sent to SPAMS in Outlook: Why ?

Emailings created from Zoho campaigns sent to SPAMS in Outlook: Why ?

Hello everyone,

We're trying to create a mailing campaign for mid-august in our company. But after few tests using Zoho campaigns, sending our mailings, they've been all put on SPAMs in Outlook. And we don't know why...
My colleague did a test one month ago using the same process and using his or other contacts from Outlook and it worked. But now its automatically put on SPAMS...

Does this issue happen to some of you before ? Any clue for how can we solve this problem ?

Thank you for your help :)