[leephillips]

I don’t see why you haven’t already answered your own question with the pi. Just ssh in to the pi and send your emails, no?

[trevi]

I would like to be able to just send an email from anywhere, eg my mail client on the phone, without any extra steps

[leephillips]

Maybe:

Set up the smtp account on the phone to be the pi. Set up postfix on the pi to use the ISP as a smarthost.