CiviCamaign is not an extension but a component (ie it's part of the core), you can enable/disable it from (administer>system settings>Components)

and disable it.

This being said, I'd be surprised it's the issue, lots of install have civicamaign enabled and working email receipts

CiviCampaign is not an extension but a component (ie it's part of the core), you can enable/disable it from (administer>system settings>Components)

and disable it.

This being said, I'd be surprised it's the issue, lots of install have civicampaign enabled and working email receipts