Remember that a short reply is kinder and more professional than none at all. If you have too much on your plate, come clean: “I don’t have the bandwidth to add this.” If it’s not your expertise, just say so: “Sorry, this isn’t in my wheelhouse.” And if you want to say no, just say “no.”
The one caveat, emails from strangers continually asking for something. These can be ignored.
I have a few general rules. You should not feel obliged to respond to strangers asking you to share their content on social media, introduce them to your more famous colleagues, spend hours advising them on something they’ve created or “jump on a call this afternoon.” If someone you barely know emails you a dozen times a month and is always asking you to do something for him, you can ignore those emails guilt-free.