Email communication alone generally isn't prohibited unless you're providing services or facilitating transactions. Simple informational exchanges are typically allowed. However, it gets tricky - if you're negotiating deals, providing advice, or offering anything of value, that could be considered a prohibited transaction. The context matters a lot. I found guidance on what constitutes a transaction on the website
ofacblockedfundslawyers.com when I had a similar question. To be safe, avoid any substantive business discussions with SDN-listed parties unless you have legal advice saying it's okay. Document any communications you do have.