I am having trouble activating a user on BES. All other users are fine and I have included the relevant portion of the log file.

I have also ran IEMSTest.exe and all results came back successful. What am I missing?

[40682] (07/29 08:46:30.984):{0x1B84} AddPagerToList(C Foster) - UserId=217
[30041] (07/29 08:46:30.984):{0x1B84} Starting handheld for C Foster
[40704] (07/29 08:46:30.984):{0x1B84} MAPIMailbox::MAPIMailbox(2) ServerDN=/o=USA/ou=Exchange Administrative Group (FYDIBOHF23SPDLT)/cn=Configuration/cn=Servers/cn=MAX/cn=Microsoft Private MDB, MailboxDN=/o=USA/ou=Exchange Administrative Group (FYDIBOHF23SPDLT)/cn=Recipients/cn=cfoster
[40739] (07/29 08:46:30.984):{0x1B84} MAPIMailbox::MAPIMailbox(2) - ResolveName[1] success for DisplayName='/o=USA/ou=Exchange Administrative Group (FYDIBOHF23SPDLT)/cn=Recipients/cn=cfoster'
[40430] (07/29 08:46:31.000):{0x1B84} {cfoster@***.com} MAPIMailbox::MAPIMailbox(2) - Mailbox opened on session 0x015a2798 (MAX)
[20116] (07/29 08:46:31.000):{0x1B84} OpenRootFolder - Mailbox::m_pMDB->OpenEntry (0x80004005) failed
[40720] (07/29 08:46:31.000):{0x1B84} MAPI call failed. Error 'The client operation failed.', LowLevelError 1209, Component 'Microsoft Exchange Server Information Store', Context 1281
[20080] (07/29 08:46:31.000):{0x1B84} {cfoster@***.com} MAPIMailbox::Construct - Open Root Folder failed
[20265] (07/29 08:46:31.000):{0x1B84} {cfoster} MAPIMailbox::~MAPIMailbox - DeleteAllDeviceSearches (0x00000000) failed
[20154] (07/29 08:46:31.000):{0x1B84} User C Foster not started
[50079] (07/29 08:46:31.000):{0x1B84} 1 user(s) failed to initialize

Looks like the BES service account doesn't have the correct permissions to access that user's mailbox.
Is that user a domain admin or a member of any protected AD groups?

Also, see here:
http://www.port3101.org/featured-bla...t-log-bes.html

They are a domain admin, but so am I, and when I tested on myself I was able to activate just fine.

A BES user can't be a domain admin.
Before you give me the "well I'm a domain admin and mine works fine" speech... please search the forum. This is one of the most discussed topics for BES Admins.

Remove the user from the domain admins group, and any protected AD groups.
Wait about a half hour, re-apply the Send-As permissions, and you should be fine.
Don't add the user back to the Domain Admins group. It will only fail again eventually.