Help / Advice: hints on "email archiving"?

[Thomas Kleindienst]

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Hello list

we're running a licensed Kolab instance in our small business since approx 1½ years now. We're generally quite satisfied but Roundcube seems to be getting slower and slower with an increasing amount of emails. Each user now has approx 40-50000 mails with about 50-70Gb per user. We'd like to archive the older mails: 

Question:  What is recommended for email archiving?

Thomas