worry about emails security

From this year we email accounts and tax return to clients as attachments instead of posting to them. My question is whether emails are not secured and what can be done to increase security (obviously there is too much sensitive data sent over the internet and we need to make sure that we are protecting client's data over the transfer re identity fraud etc). Are there any proven and practicable/easy ways to transfer the data with maximum security (eg any software for encryption etc) and to avoid any problem with data processing act (i am not sure what the act says on this). I know you can put passwords on pdf files but this is a bit of a hassle and difficult for some clients to implement as well as for us to administer.

Furthermore, my other related question is, is it acceptable for the client to sign the tax return and accounts and email scanned signature instead of posting the docs by normal post? Are we covered for obtaining client's authorisation for online filing of their return if signature is not the original but a scanned one?

thanks