I like the idea, this would be like you said a time-based expiration with a message cap of say 25. You would turn on a global setting to "automatically store sent messages".
Further features would include increasing the amount of pm's kept in the sent folder vs fg (monthly) and length they stay there before they auto delete vs fg (monthly).
Default: Everyone could get say the last 25 messages, and they last 14 days before they're auto deleted. This would increase to keeping the last 100 messages and they would last for 30 days if you were
Verified.
Addons:
Increase storage size by 25 messages vs 150fg/month (would work the same as the LS Item Vault)
Increase storage time by 7 days vs 200fg/month (would work the same as the LS Item Vault)
Messages stored in your "Sent backup storage box" would
NOT count towards your allotted pm slots.
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Main reason why I like this is you would then have the messages required should you need them if a S/A case came up
without needing to remember to click the save pm check box.
I like this. Voted yes.