It could be anything in the email, even something seemingly innocuous in the signature or footer.
post the complete error message here and the forum team will use the unique token (seemingly a random character string) to identify the cause however be aware it can take then a few days to work through to your post
If you wish to troubleshoot the issue yourself try:
dividing the email content
sending each half to yourself
check your spam folder for the half sent in step 2 and repeat from step 1 until you have identified the content that is triggering the spam content warning.
Message could not be sent to the following recipients: <email address here>] (554 - 554 5.2.0 SMTP602 Message contains Spam Content ;id=1WVJhGzApO7On1WVJh66f1;sid=1WVJhGzApO7On;mta=smtp1.tb;d=20190306;t=142237[CET];ipsrc=172.25.160.142; )
This is my error message. Can anyone help? In the meantime I will break the emails down and try and see what is causing the pain.