There is a risk of a hacker intercepting your e-mails. This risk is more significant if you are transmitting material such as tax documents which show your social security number and possibly other things like your bank account number. If you send the information by US mail, there is a risk that someone other than the addressee will get it.
When I transmit a tax return I send it as a pdf file created by PDF Factory, which you can buy for under $ 50. Instead of printing a paper copy of the return I use PDF factory as the printer. PDF Factory enables me to use a security code, so I use the social security number of the recipient. That way the only way a hacker is likely to get into the e-mail is if he already knows your social security number. The PDF factory also enables me to use a master password which enables me to open the documents without using the recipient’s social security number. I never tell anyone what that password is.
If, for example, you are e-mailing tax data to your CPA, you should consider buying PDF Factory or a similar program. In addition to protecting sensitive material, it is also very useful for other things. I have it as my primary “printer,” so I always print things as a pdf. If I accidentally print a 20-page document by mistake, I don’t waste 20 sheets of paper. I just look at the PDF and delete it. If I really want to print a paper copy, I just select “print” within the document, and then I can select any printer I want to use. It defaults to the printer I normally use. Any time you send a document, it is much more readable if you send it as a PDF file rather than a JPG file. A lot of people have scanners that only create JPG files. When I get JPG files with documents, I finally found a way to convert them to PDFs before I print them as paper copies. Before I discovered how to do that, I had a lot of trouble with information I received in JPG format.
Of course there is no absolutely safe way to transmit your documents. To eliminate the risk of interception, you would need to hand-deliver any information that could be used for identify theft. A lot of my clients live 20 or more miles from me, so hand-delivery is inconvenient and also expensive as the price of gasoline rises.
One other option would be to fax the information. That is safe unless you accidentally send it to the wrong fax number. I sometimes get faxes intended for other people who have a fax number one digit different than mine.