A significant set of comments here approach this problem from the server and sysadmin side. None of them approach the problem from the user interface or usability side. How can your implementation be successfull if your users (up to 1 million) are unhappy with the way they are required to use it, or it's unusable to them (very common for those not technically inclined as you are almost certainly to have with that many email accounts)
First you need to determine how the users will access their email, and how they are going to use it. Will this be webmail, client app, PDA, etc.
Then you need to determine what the user requirements are:
Calendaring support (this will be nasty if application integration is required)
Shared contacts
Webmail (and what browsers will be used)
SSL/TLS
SMTP Authentication
Roaming users (anyone not on the intranet)
IMAP, IMAPS
POP3, POP3S
Also what applications they will be using to connect to this system such as: emacs, pine, mutt, thunderbird, outlook express, MS outlook, opera, evolution, mail.app, etc.
If possible try to enforce a policy restricting the use of email clients to a small subset, but do however remember that there may be users on Mac's, **nix PC's, Windows PCs, and potential others. (NB: Avoid allowing Outlook Express if you wish to use IMAP)
Determine your security requirements for the mail system. Is everyone required to connect using SSL encrypted links?
Determine your minimum service levels required (99.9% uptime or higher, do note that every 9 beyond the first 3 can be expected to double the cost of the solution)
Determine support levels for hardware with respect to warranty, part availability, technician availability etc.
Determine backup requirements, are you required to be able to restore individual emails, individual mailboxes, all mailboxes, and how many levels of backups are required? Do you need to be able to restore emails deleted 4 months ago
Quota requirements, are there limits on the size of a persons mailbox, can this be customized, are there limits on the size of an email a user can send, and the same for receiving. Will you allow a user to store 2+GB of email on your system?
Determine other legal requirements, such as a requirement to be able to retrieve any email sent through the system for auditing/legal purposes
Determine effectiveness of antivirus filtering and how many levels of antivirus filtering will you require to ensure robustness and the correct level of user protection?
Determine level of spam filtering required (generic, user specifiable, with headers, without headers,
Do you require mailinglist or distribution list requirements (mailman?)
How many physical sites will be accessing this mail system (one office? multiple branches)
Will users be able to customize their mail settings (enable/disable bayesian spam filtering, custom antispam rules, setting of spam thresholds, autoresponder messages, out of office replies, disable/enable spam filtering, disable/enable antivirus filtering)
What level of redundancy are you required to have? Do you need to provide redundant systems even if one datacentre is disconnected (somehow)
ie. main datacentre you use in the UK is disconnected for some reason outside your control, do your roaming users in the UK still need to be able to access their email without any loss through an alternative backup mail system in the US?
Can your users be split up into multiple sub-domains? ie. production, hr, finance, lists, support, technical, development, etc. And will they notice or can you hide it from the user with a simple server-side rewrite.
How are you going to measure the performance of the system once in place. wrt disk space, amount of connections, upti
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A significant set of comments here approach this problem from the server and sysadmin side. None of them approach the problem from the user interface or usability side. How can your implementation be successfull if your users (up to 1 million) are unhappy with the way they are required to use it, or it's unusable to them (very common for those not technically inclined as you are almost certainly to have with that many email accounts)
First you need to determine how the users will access their email, and how they are going to use it. Will this be webmail, client app, PDA, etc.
Then you need to determine what the user requirements are:
Also what applications they will be using to connect to this system such as: emacs, pine, mutt, thunderbird, outlook express, MS outlook, opera, evolution, mail.app, etc.
If possible try to enforce a policy restricting the use of email clients to a small subset, but do however remember that there may be users on Mac's, **nix PC's, Windows PCs, and potential others. (NB: Avoid allowing Outlook Express if you wish to use IMAP)
Determine your security requirements for the mail system. Is everyone required to connect using SSL encrypted links?
Determine your minimum service levels required (99.9% uptime or higher, do note that every 9 beyond the first 3 can be expected to double the cost of the solution)
Determine support levels for hardware with respect to warranty, part availability, technician availability etc.
Determine backup requirements, are you required to be able to restore individual emails, individual mailboxes, all mailboxes, and how many levels of backups are required? Do you need to be able to restore emails deleted 4 months ago
Quota requirements, are there limits on the size of a persons mailbox, can this be customized, are there limits on the size of an email a user can send, and the same for receiving. Will you allow a user to store 2+GB of email on your system?
Determine other legal requirements, such as a requirement to be able to retrieve any email sent through the system for auditing/legal purposes
Determine effectiveness of antivirus filtering and how many levels of antivirus filtering will you require to ensure robustness and the correct level of user protection?
Determine level of spam filtering required (generic, user specifiable, with headers, without headers,
Do you require mailinglist or distribution list requirements (mailman?)
How many physical sites will be accessing this mail system (one office? multiple branches)
Will you be requiring a support ticketing system? (example: RT from http://www.bestpractical.com/)
Will users be able to customize their mail settings (enable/disable bayesian spam filtering, custom antispam rules, setting of spam thresholds, autoresponder messages, out of office replies, disable/enable spam filtering, disable/enable antivirus filtering)
What level of redundancy are you required to have? Do you need to provide redundant systems even if one datacentre is disconnected (somehow)
ie. main datacentre you use in the UK is disconnected for some reason outside your control, do your roaming users in the UK still need to be able to access their email without any loss through an alternative backup mail system in the US?
Can your users be split up into multiple sub-domains? ie. production, hr, finance, lists, support, technical, development, etc. And will they notice or can you hide it from the user with a simple server-side rewrite.
How are you going to measure the performance of the system once in place. wrt disk space, amount of connections, upti