Helping people with computers... one answer at a time.

I'm regularly asked if having a lot of email has an adverse affect on overall computer performance. Not really, but there are things to be aware of.

Can my email slow down my computer? I mean in my case Windows Live Mail, because I use space and memory to store received mails and very often sent mail remains for long periods of time before I delete them, so basically all I want to know is does the Windows Live Mail or Outlook Express or Windows Mail slow down a computer in any way at all or not?

No, not really.

At least if I understand your question correctly - accumulating a lot of email on your PC doesn't necessarily slow it down.

However... there are scenarios that might affect a few specific things.

Lots of Email is Fine

Simply accumulating a lot of email over time isn't typically a problem. It certainly won't slow your PC down in the way that most people who ask this question are concerned about.

So keep all you want - as much as your hard disk will hold, if you like.

I certainly have gigabytes of email.

However, the are scenarios that you should at least be aware of.

"Honestly, having lots of email isn't really a big deal any more."


Most email programs, including the ones you mention as well as many others, will perform periodic maintenance on the files that they use to store your email. Typically it's some kind of "compaction" where the empty space formerly used by deleted emails is removed and the overall disk space required to hold what remains is reduced.

Some email programs maintain separate indexes into your email so as to make the email faster to access or search. Maintenance might include rebuilding or updating those indexes.

Naturally the more mail you have the longer this maintenance can take.

If you happen to be using your PC while the maintenance is happening, or especially if you're attempting to use your email program then yes, things can be slower for however long the maintenance process lasts.

Bugs & Limitations

I normally don't call out bugs as something to be specifically concerned about, but there are two specific situations that warrant it:

Versions of Outlook (not Outlook Express, they're unrelated) prior to Outlook 2003 have a 2 gigabyte limit on the "PST" file that's used to store all of your information. The problem is that the limitation was not really enforced, so PST files could easily grow larger than that without notice. Unfortunately when that happened the program sometimes misbehaved in different ways - crashing, losing email, hanging or just being extra slow. Upgrading the program to a newer version didn't help, the PST was not automatically updated to a newer format, so that had to be done manually.

Outlook Express's storage mechanism, while completely different and unrelated to Outlook's, appears to have similar problems when individual email folders exceed 2 gigabytes. In this case, since Outlook Express has been discontinued there's no real remedy.

So having a lot of email in Outlook or Outlook Express could absolutely cause problems - performance issues being perhaps the least of your worries.

Your Disk

I'll admit this is a bit of a stretch, but it's worth mentioning: the more email you have the more important having a defragmented hard drive is, and the higher the potential impact of the disk speed.

As I said above, many email programs use indexes to optimize their ability to access your email quickly. The more email you have the larger those indexes become. If they are fragmented then the amount of time required to access them increases. The same is true for the email itself, but ultimately in most programs that use indexes of some sort it's the index that's actually accessed most often.

Similarly, the more data your email program has to wade through to get at your email the more important your disk speed becomes in how quickly your email program can respond.

The Net Effect of Lots of Email

Honestly, having lots of email isn't really a big deal anymore. Assuming you have plenty of disk space the impact on the rest of your system is really minute.

If there is an impact it's really only on your email program itself and not that of the rest of your system.

Oh, and your backups (you are doing backups, right?) may be larger and take somewhat longer, but that's both expected and incredibly important to avoid losing all that email due to a disk failure.

Article C4717 - January 21, 2011 « »

Leo Leo A. Notenboom has been playing with computers since he was required to take a programming class in 1976. An 18 year career as a programmer at Microsoft soon followed. After "retiring" in 2001, Leo started Ask Leo! in 2003 as a place for answers to common computer and technical questions. More about Leo.

Not what you needed?

Alex Dow
January 25, 2011 11:00 AM

Do you really need to keep most of the e-mails?

How often have you gone back and opened ancient e-mails?

Could selective, progressive Back-ups help?
That is Backup older e-mails in yearly groups or similar, to an external HDD, leaving more space on your working HDD.

Simon Boston
January 25, 2011 11:14 AM

In #267 you say that Outlook Express has been discontinued. I am using OE, so should I change to Outlook, and if so how do I transfer my folders (my filed emails) to it?

James Hillier
January 25, 2011 1:00 PM

Personally, I believe it is not best practice to use your email client as a storage area for emails. If you do not regularly backup email client settings/folders separately, any minor corruption and it is possible to lose them all.

Much better idea to set up a new folder in (My) Documents and name it something like 'saved emails' and save them there. You can also create as many sub-folders as necessary to cover categories/contacts.

Not only would they be safer using that method, they would also automatically become part any normal backup strategy.


January 25, 2011 2:36 PM

While this may seem a bit paranoid, one good reason to delete old correspondence is to minimize time consuming aggravation should you become involved in litigation. This mainly applies to business correspondence.

It is legitimate for an opponent to demand ALL emails even remotely related to subject being disputed. A typical problem that might ensue is if you have several drafts of a document, all sent by email. Some drafts might contain items that you considered using, but discarded. In front of a judge or jury, that will be very difficult )perhaps even embarrassing) to explain.

Note-there are legal requirements to retain some email correspondence. One should consult an attorney when in question (I am not one) to develop a policy about what to retain. Then get rid of everything elso periodically.

January 25, 2011 3:45 PM

If OE is discontinued is there a way to remove OE? Should OE be removed?

I don't see a reason to. (It's difficult, at best, anyway.)

Kathy P.
January 31, 2011 10:43 AM

Hi Leo:

Along this same line, I use Hotmail as my main email; however, often something will come up and it opens Windows Live Mail, which seems to have all my Hotmail email in it, too. Does having the email in these two places take up more space and how do I not have it in both of them? Thanks, Kathy

I believe Windows Live Mail is simply accessing your email on Hotmail via a "live" connection (IMAP, for those that know). It takes up a little space on your computer but not a lot. It's a convenient way to be able to access it both ways as you like and have everything stay in sync.

Bill- Vancouver Island, BC Canada
February 3, 2011 11:40 AM

Great item; you should see the size of the "Ask Leo" folder I have saved in OE. And Randy Cassingham's "TiT" as well.

err, BTW Leo, the plural of "index" is "indices".

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