what is the max. Mailbox Size, Elements count (Exch. SE) ?

Hi @Stefan K 

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In Exchange Server Subscription Edition, there is no hard limit on the number of items in a mailbox.  

Mailbox size:  

-Exchange SE enforces mailbox limits by size (GB).  

-These limits are configured using mailbox quotas.  

-Quotas can be configured per mailbox or inherited from the mailbox database, with the effective limit being the lowest applicable value.

-There is no fixed hard-coded maximum mailbox size in Exchange SE, the effective limit depends on:   

• Configured mailbox quotas  
• Mailbox database limits  
• Available storage and performance considerations  

-Exchange SE does not enforce a maximum number of items (emails, calendar items, folders) per mailbox. A mailbox can contain very large numbers of items as long as it stays within the configured size quota and does not exhaust system resources. However, very large item counts, especially within a single folder, can lead to performance degradation in Exchange and client applications such as Outlook.

-Mailbox databases apply limits at the database level, not per mailbox. Database limits are size-based and determined by edition, configuration, storage architecture, and recovery and performance considerations, rather than by item count.

-The Managed Store limits documentation applies to Exchange SE but does not describe mailbox storage limits. It describes runtime resource limits such as sessions and open objects, which are designed to protect server stability and do not limit mailbox size or total item count.

References: 

• Configure storage quotas for a mailbox | Microsoft Learn 
• Exchange Server storage configuration options | Microsoft Learn 
• Manage mailbox databases in Exchange Server | Microsoft Learn 

I hope this clarifies your question. 

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