What are .ost/.dat/obi files and may I delete them?

[BudV]

Running OL2007 under Vista Home Premium SP2. I have two mail accounts with
a pop3 server.

I have one two-months old .ost file for each of my mail accounts (55MB and
103 MB) and I don't know why they were created. While I'm at it, I also
have a 1KB extend.dat file from four days earlier, and a current
mailbox.sharing.xml.obi (2KB). In the interest of neatness, I would be happy
if I could just have my one big .pst file (1.1GB).

How much of this can I throw out?

 

[Shawn76 via OfficeKB.com]

the .ost files are mirror copies of the data stored in the mailbox on your
Exchange server. If they are 2 months old, you can delete them if you like.
Every time you open Outlook and connect to the Exchange server,an OST file is
created. OST files allow you to be able to work in Outlook offline, then when
you go online, it will sync with Exchange.

the extend.dat file is a binary cache of file extensions. Deleting it may
cause problems with Outlook. Just leave it

the sharing.xml.obi contains RSS subscription names. You can delete it, but
the next time you open outlook, it will be recreated.

 

[Brian Tillman [MVP-Outlook]]

the .ost files are mirror copies of the data stored in the mailbox on your
Exchange server. If they are 2 months old, you can delete them if you like.
Every time you open Outlook and connect to the Exchange server,an OST file
is
created. OST files allow you to be able to work in Outlook offline, then
when
you go online, it will sync with Exchange.

If you're no longer connected to the Exchange server but the account is still
in the mail profile and use Outlook offline (because, say, you changed jobs
and no longer have access to the server but wish to continue to see the
messages, contacts, etc. you had), deleteing the OST will screw you over.

the extend.dat file is a binary cache of file extensions. Deleting it may
cause problems with Outlook. Just leave it

Deleting it shouldn't cause problems. Since it's a cache, it will be
recreated from the registry when Outlook starts again.

 

[Bud Vitoff]

One problem is that I have no familiarity with Exchange servers. I do know
that I have no need for their sophistication and life is much simpler for me
and my multiple mail accounts when I stay in a POP3 environment.

I believe the dates associated with these files coincide with when I
switched from att.net to live.com and I was stuck with a MAPI (IMAP? I
forget which) server and the problems it gave me in OL. Diane simplified my
situation my having me use pop3.live.com and life was good again.

So now that it appears that some of these files may just be leftovers from
that period of time, does that change your advice on getting rid of them?

 

[Brian Tillman [MVP-Outlook]]

One problem is that I have no familiarity with Exchange servers. I do know
that I have no need for their sophistication and life is much simpler for me
and my multiple mail accounts when I stay in a POP3 environment.

I believe the dates associated with these files coincide with when I
switched from att.net to live.com and I was stuck with a MAPI (IMAP? I
forget which) server and the problems it gave me in OL. Diane simplified my
situation my having me use pop3.live.com and life was good again.

So now that it appears that some of these files may just be leftovers from
that period of time, does that change your advice on getting rid of them?

If you were using the Outlook Connector (which will employ an OST as a cache,
just like Exchange) and you've switched to POP access, then, yes, you can
safely delete the files because you don't need them.