I too have resorted to email in between sessions and found that if I could not get right back to my clients he/she would be upset. I explained that I do not always have the time to email immediately and that I would get back to them as soon as possible. Emailing works well with a selected few.
I do agree that delayed gradification and containing material until the next session is necessary.
Confidentiality is a concern for me as well. My main thrust is basically to reasure my client that I have heard what they are saying and it will be addressed in person during our next session. I never give alternative solutions via email. Being understood and validating them seems to sooth their worries until our next session.
Thank you for bring up an issue that needs to be explored in more detail.
Sincerely, Leslie Dale