365 business email : when is email mbx too big/unusable

Hi There

Business 365 emailAt what stage does the email mailbox size become simply too big to be workable , and start causing issues

Ive seen some say 10Gb The issue Im seeing, with users on DSL (no fibre) , with 20Gb of 365 cloud stored email , the sync starts to get unworkableFix an issue with the sync, then wait days (yes days) for it all to sync/fully download. In the meantime the are issues with speed & searchesThen wait till the next sync fail & start again.

So at what size do you say, too much email, archive it out ?Especially on (slow) ADSL

Cheers