Yes. You can in some circumstances be held liable for defamation if you make comments in an email exchange that are damaging to someone else’s reputation.
The concept of defamation is to provide a remedy to someone who is a victim of damaging comments that are made public or are distributed in some way. So if you make negative comments about someone in an email, and address it to only one person then chances are it will be fine. However, if you are sending emails to multiple people with material that is defamatory, then yes, you may be held liable. Anyone involved in a defamatory comments’ publication or distribution may also be held liable, so if you receive an email that might be defamatory, don’t pass it along!