First, I have no solution Second, my suggestion is to first verify that you are authenticating at your SMTP server, as that is a common cause and easily corrected.

Well, if the mail doesn't have any links in it, it will be sent

Another possibility might be to check the URL for blacklist. Here is one site that does that: https://dnschecker.org/ip-blacklist-checker.php

It is a valid microsoft teams link every time.

SMTP error from remote mail server after end of data:

Typical cause for this error is when the recipient's spam filter blocks emails sent by the sender. OR due to the configuration made on the SPF record that denies or adds specific domains to blocklists.

SMTP error 550 usually appears when there is an issue with the authentication of the SMTP client and the SMTP server.

Hence a valid comment posted by david to check smtp server.

550 High probability of spam Content in email triggers the server filter - for example by a link to a site that is often mentioned in spam.

You have managed to send an email with no links so this problem is not your IP address nor email address.

But is it any email with any link or just a microsoft teams link or just to a specific recipient?

Send a test email with a simple link to another website. Such as a link to this webpage: https://support.mozilla.org/en-US/questions/1410612

Are you only getting rejections if the link is microsoft teams link?

I located some links that may provide some insight:

https://answers.microsoft.com/en-us/msteams/forum/all/teams-webinar-invite-either-goes-to-spam-or-gets/c8890935-3de2-4b19-bbdb-207d47ce50bb

https://learn.microsoft.com/en-us/answers/questions/1045372/teams-invitation-emails-marked-as-spam