GDPR can only apply to any personal information in the email. For example, if a Councillor emails me to say "The pothole in North Road has been repaired" the only personal information is the authors details. Depending who the author is, this could be redacted if necessary before the email is forwarded to a resident.
Also, as Letsbehonest alludes to, there are 6 lawful grounds for sharing personal information of which consent is only one.