Doodledog
I think it is unreasonable to email staff using their personal addresses. I used to work quite flexibly and would send emails late at night (to work addresses) but with no expectation that there would be any response out of office hours. It just suited me to send them when I was working in the evening, which I often did so that I could do other things if I had no appointments during the day.
I wouldn't acknowledge the email at all from a personal address. I would reply when I got back to work (using my work email) saying something on the lines of:
Dear Boss
I noticed an email from you on my personal address during my annual leave, and forwarded it to my work address to deal with it on my return.
Then say what you want to about the meeting.
It would be better if you used my work address for work-related matters, as I don't always check email on holiday and it could get lost when I clear out the numerous messages I get every day, and it was only by chance that I spotted this one.
Also, my husband/children/cleaner/dog has access to my private account, and I can't guarantee that messages to this address will remain GDPR compliant.
Best wishes
Manna66
Ooh, I like this. Firm but professional, covering everything.