Based on my research, there are several possible reasons:
One possible cause is that the Teams invitations are sent by the Exchange Online service, which may not have proper email authentication settings for your domain, such as SPF, DKIM, and DMARC. These settings help prevent spoofing and phishing by verifying the sender’s identity and domain. If these settings are missing or incorrect, some email services may flag the invitations as spam. For more details, you may refer to the article.
Another possible cause is that the recipients have strict spam filters or rules that block or move the invitations to the spam folder. This may happen if the invitations contain certain keywords, attachments, or links that trigger the filters or rules. You can ask the recipients to check their spam settings and whitelist your domain or email address. You can also try to avoid using words or phrases that may be considered as spam, such as “urgent”, “guaranteed”, “free”, etc.
If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.