Some rules to consider...
An online classroom is still a classroom therefore appropriate classroom behaviour still applies.
Consider the privacy of others and ask permission prior to giving out other student email addresses.
No offensive or inappropriate material.
Avoid language that may come across as strong or offensive, language that could be easily misinterpreted in a written format.
Read before you send, there’s no taking back a comment that has already been sent.
Keep attachments small. If it is necessary to send multimedia, ensure they are of a small file size to be easily downloaded by users.