I just made a donation online to Macmillan Cancer Care in memory of an old friend who died recently. The small print says that they'll put me on a mailing list and send me stuff, unless I write to ask them not to.
Thinking that I get enough charity mail already, I emailed them to ask them not to communicate. So I did enjoy their reply, from one of those 'no reply@' email addresses:
"Thank you very much for contacting Macmillan. A member of the Supporter Donations team will be in touch with you within 2 working days."