Compacting Outlook 2010 OST results in old emails being re-sent

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Thomas Inman

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Outlook version
Outlook 2016 32 bit
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Outlook.com (as MS Exchange)
Hello Diane and fellow readers,

I have a customer running outlook 2010, connected to an offsite company exchange server, and I am compacting the OST now. The customer's OST file is 45GB!! Hence compacting is still running as I write this, 18 hours later.

The customer reported to me this morning that she was told people are receiving old emails from her, and I have never heard of compacting an OST causing this to happen.

This customer's Outlook OST is the largest I have seen since Microsoft changed the size limit from 20GB to 50GB.

My questions are, can compacting actually trigger old emails to be re-sent? And, how long should I continue to wait for compacting to complete? Or should I cancel compacting?
 
compacting the OST
When it is an ost, it's not really necessary to compact it - you can delete it and let outlook resync. If the mailbox is huge and the internet connection slow, you can check the mailbox size online before deciding which route to take...

and no, i have never seen this reset in old emails being sent. it also shouldn't trigger read receipts being sent back - unless you empty deleted items before compacting.

If they are actual messages being resent, i would 1) verify in OWA that sent messages aren't stuck in the outbox and 2) delete the ost.
 
The user has a 50 down/10 up connection with Comca$t, however I opted to not delete the OST because it's 45GB, and figured that would take a few days to download again.

She is relatively close to the maximum size for an OST, which I believe is 50GB, so I am hoping that her OST will actually shrink in size. I don't think it has ever been compacted before, spanning at least 5 years. She isn't a huge fan of technology, especially when it comes to computers, so I am going the extra mile to help her any way possible.
 
if it is not erroring for her, then it is 50 GB (2010 started out with a 20GB default, but it was increased in an update) - and you can raise it higher, although microsoft doesn't recommend going too much higher.

Compacting has the potential to reduce it 20%, so you may recover a lot of space. (Outlook is supposed to automatically compact when there is 20% white space - which is harder to hit when the file is huge.)

My ost is 10GB and i deleted it earlier today hoping to fix a problem with rules (it didn't). It took about 2 hours to download from office 365 on a 100mbit connection.
 
Update: Compacting had not finished after waiting 24 hours, though the progress bar was still moving toward completion (about 80% done on a 45MB OST file). This user needed her machine back asap, so time was up waiting for compacting to completely finish.

What improved significantly was no more spinning wheel mouse pointer waiting on background operations in Outlook 2016. Navigation is quick once again. I did not have an chance to see the resulting file size after letting it compact for that long a time, but I consider this a successful repair based on the user's positive feedback alone.

Regarding the issue mentioned earlier about the user reporting feedback from colleagues that received 'old' emails from her during the compaction: this appears to be a result of duplicates stuck in the Outbox for who knows how long, that were suddenly able to be sent out. I don't think the issue happens as a result of compacting necessarily, but it was a side effect in this user's case. No such problem exists now, and only a few of her colleagues received duplicate old emails - not everyone in her contact list. So, I believe Compacting an OST file is not harmful, but be prepared for the length of time it takes to complete on an unusually large file.
 
I consider this a successful repair based on the user's positive feedback alone
Absolutely. My guess is there was either a huge email she deleted (into the GB in size) or a ton of little ones that needed cleaned up. Can't really guess on the size based on how long it took for that reason - but i'm expecting it went down a lot - probably close to half, if not more.

It may have been faster to re-sync it - this is one reason why microsoft recommends deleting ost files....
this appears to be a result of duplicates stuck in the Outbox for who knows how long
This may have been a big reason why outlook was so sluggish but they should not have been stuck there to begin with.
 
I think your guess on the reasons behind the dramatic speedup in Outlook after compacting the OST is correct.

On another customer system, I did this process differently, deleting the OST, and waiting for a full re-sync. His mailbox was not as large (over 20GB), but still took almost 24 hours to download everything. I watched the count as emails came in, and it was through short bursts with long pauses in between that the re-sync slowly finished. That was with an Office 365 Business plan/Exchange mailbox about 12 months ago.

I haven't figured out the rate O365 transfers data, or more specifically, the pattern, but from my own observations, it seems to be a start/wait/start process. This seems strange to me considering Microsoft has virtually unlimited bandwidth.
 
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