It is often the case that the more convenient something is, the more data protection risk it might present.
The use of email address auto-complete (sometimes called auto-fill) is an example of this. Sending an email containing personal data to the wrong person, can be a common cause of a data breach, but the risk can normally be reduced by turning off auto-complete. However, it is a very convenient feature, provided we are careful and check it’s the email address we want.
Whichever approach you choose, we would encourage you to agree a charity-wide position, document it, and make everyone aware. You should also consider how this affects people using their own devices (BYOD) as well as one that you supply.