It may be legal to publish the sender's text. The risk is a copyright infringement suit, but you might be able to avail yourself of the fair use defense since you are "commenting" on the work. Your use seems to be "transformative", not just copying and redistributing, but creating something now where the original – in a modified form – is ...
If you are relying on consent as the legal basis for this processing of personal data, then it MUST be opt-in. Pre-ticked checkboxes are not compliant.
If you are relying on a legitimate interest though, then an opt-out solution is OK. What falls under a legitimate interest can be fairly flexible, but you are required to show that this legitimate interest ...
Article 4(11) says:
‘consent’ of the data subject means any freely given, specific,
informed and unambiguous indication of the data subject’s wishes by
which he or she, by a statement or by a clear affirmative action,
signifies agreement to the processing of personal data relating to him
Recital 32 says (my emphasis):
Consent should be given by a ...
The first legal impediment is that you don’t own the copyright in the email so copying it without permission is prima facie copyright violation.
You may have a fair use defence but that is not clear. You don’t have a fair dealing defence. You may be able to argue that you have an implicit licence to make copies of correspondence but I doubt that ...