Hello @nasim.nazanin88 – if you and friends use national iranian e-mail servers with Delta Chat, then your messages will become end-to-end encrypted between you. If you also use it to e-mail non-delta chat users then those communications will not be encrypted, usually.
For DC users to protect messages against potentially hostile e-mail servers it’s best to use QR codes for setting up contact with each other, and then create so called “verified groups”. For more technical background, you can read here 2. Securing communications against network adversaries — Counter Mitm 0.10.0 documentation