Yes, replies to your emails are encrypted if the email service you are using supports encryption protocols such as TLS (Transport Layer Security). However, the level of encryption can vary depending on the service provider and the settings you have configured.

Even if the recipient uses a device to relay through a server that isn't enforcing encryption? For example, what if a recipient received a message at a properly secured domain but they receive this from an IMAP connection on a personal cell phone that is set to use a different outgoing SMTP server that isn't properly secured? I know it sounds far fetched but I just feel it is an area where you should be clear to your customers that you cannot enforce proper security on recipients. Once the email is received those recipients very well could circumvent the policies that Paubox has put in place and therefore it is important to inform recipients to be careful with replies, or use a more strict transmission process (Like Paubox's own secure weblink)...