I have a strange problem. I want to use the deal specific address (dsa) in order to add some mails from an archive mailbox (not connected) to the deal history. 

The strange thing is that some arrive and some don't depending on the sending organization. I have several email account so I could check that sending from account A works fine while sending from account B does not. 

Question: What am I missing? Is there a hidden Anti-Spam feature?