You should not rely on receiving an email reminder before your UK ETA expires.
A UK ETA is generally valid for 2 years from the date of approval, but if the passport used for the application expires earlier, it also expires with that passport.
Therefore, instead of relying only on an email notice, it is important to check the validity of your approval and your passport expiry date before traveling.
If it has expired or your passport has changed, you cannot use the existing UK ETA and should apply for a new UK ETA before traveling to the UK.