How can e-mail security be improved?
By encryption. It causes the e-mails to only be read by the specified receiver.
I deliberately try to be vague and obscure in my e-mails. I do not mention names or figures. Any potential spying will be thwarted.
E-mails can be signed. This does not prevent someone intercepting, but it does not allow anyone to change the content unnoticed.
I leave the subject header empty when sending confidential data.