If you want to send an email to a conversation, use the conversation email address.
Desktop:
1. Open the conversation.
2. Click 
Settings icon on the top right corner.
3. Under the Conversation Email Address click on Copy to copy the conversation email.
4. You can now use this to send an email to the conversation.
Mobile:
1. Open the conversation.
2. Tap Settings icon on the top right corner.
3. First Tap on the Conversation Email Address then on Copy to copy the conversation email.
4. You can now use this to send an email to the conversation.