To be on the safe side, I would take the number of mailboxes you have, multiply them by the size you want to allocate to the mailboxes, and then add 25% - for domain and system overhead, to the total domain size.
This must be sufficient to cover each user's:
- e-mail boxes
- sent items
- deleted items
- created sub-boxes
- SMTP, IMAP and POP mappings from other e-mail services
- aliases - you will be required to create at least two, per IETF requirements:
- ff you are using DMARC, and forward your DMARC reports to DMARCIAN.COM, then you might also want to create an alias for DMARC as well
- personal calendar
- shared calendar(s)
- user's personal and shared files (if they are allowed to store them locally)
- external storage mappings - to externally attached file stores
- user's dictionary (some of them can get quite large)
- user's configuration and backup configuration (system generated and not under your control)
If you will be using the newsletter features, then you will need to allocate space for the mailing list.
So, if you have no mailing lists, then adding 25% of the total of the number of users, times the individually allocated space for each user, should provide plenty of space for the users and domain.
If you will have a bunch of aliases, one or more newsletter, and also be using the XMPP (chat) feature,
As I previously indicated, for our BASIC accounts, we allocate 5 gigabytes per user. To prevent problems, the domain size is set to unlimited, protected from growing too large by the individual user limits.
For our premium accounts, we set 15 gigabytes per user. Again, the domain size is set to unlimited.
We also have contracted customers who do not have any size limitations. These accounts are periodically reviewed to see how much storage space they are using, but, in most cases, these accounts also include Exchange Web Services and message archiving. The cost of the services for these accounts, and the included storage, is determined by contract negotiations.
Remember, we, each of us who provides e-mail services, is now in direct competition with GMAIL, OUTLOOK.COM, YAHOO!, COMCAST and other providers who do not impose artificially small limits and we need to consider that when we set our pricing and our user/domain capacity.
As such, the servers on which our SmarterMail installations are built must be engineered, and the storage capacity of those servers, sized accordingly.
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