Voted Best Answer
Mar 13, 2017 - 12:00 AM
Another option would be to license the users with:
Exchange Online Plan 1 (£3.00) + Exchange Online Archiving (£2.30) for a total of £5.30
about 1/3 of the E3 cost.
This approach is recommended by Microsoft:
"If a mailbox is assigned an Exchange Online (Plan 1) license (which is part of an Office 365 365 Enterprise E1 subscription), you would have to assign it a separate Exchange Online Archiving license so that a hold can be placed on the mailbox..."
Where you now need to incorporate SharePoint access as well, you could license the users with:
Office 365 E1 (£6.00) + Exchange Online Archiving (£2.30) for a total of £8.30
just shy of 1/2 the E3 cost.