Wait... so even players that are not logged in and playing are causing lag. That's disappointing... but makes me think that the first purges should be based on last login for non-Premium/non-VIP and maybe a 365 day TTL for mail for all non-Premium/non-VIP players. A 50 item limit is good, but it doesn't stop people from making a ton of free accounts just to store their items and therefore continuing the problem.
If the players that are not logged in are not contributing to the problem, then limiting logged in time to 30 minute sessions for players over 50 should accomplish the same goal without making players feel like anything has been taken from them. They can always reach out to a GM if they prefer the automated process to being kicked out ever 30 minutes.
That said, none of my mail boxes have anywhere near 50 items so no skin off my back either way.