If you receive:

An error occurs while the spelling is being checked

when spell checking in OE5, or you receive:

The spell check on this document was halted. Do you want to send anyway?

when sending a message (check spelling before sending message is configured), the spell checker, Csapi3t1.dll, was not found.

It should be in the C:\Program Files\Common Files\Microsoft Shared\Proof folder.

If it is there, then your registry entry is wrong. Use regedt32 to navigate to:

HKEY_LOCAL_MACHINE\Software\Microsoft\Shared Tools

Double-click the SharedFilesDir value name, a type REG_SZ entry, and set
the string data to "C:\Program Files\Common Files\Microsoft Shared\"

The path is one level up from the Proof folder, must end it a backslash, and must be encapsulated in " marks.