Hi,

I'm working at a large company where a lot of people only work for a short time (like several months) after which they start working for other companies. This causes quite a lot of user account changes on our Exchange environment. User accounts frequently get added or deleted.

While using evolution-ews to open my Exchange mailbox I noticed something odd. Initially evolution-ews tries to read the contents of a mail folder. Once it knows which mails are in a folder then it tries to retrieve additional information about all the e-mail addresses which are mentioned in these mails. The Exchange API function ResolveNames is used to retrieve this information. In the evolution-ews account configuration I've enabled the option to cache the contents of the Global Address List.

When EWS_DEBUG=2 is set then a huge amount of ResolveNames calls can be observed for large mail boxes (my mailbox for example is more than 3 years old and contains thousands of mails).

When looking more closely at the output I noticed that a large amount of these ResolveNames API calls return 'No results were found.' messages to requests where the user account in question doesn't exist on the Exchange server any more. This looks to me like correct behavior.

For the situation where the ResolveNames API function returns no results evolution-ews doesn't seem to do anything with this information.

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Here's an example describing the situation:

Initially the Exchange server contains the following user accounts:
- user1@domain
- user2@domain
- myaccount@domain

The account 'myaccount@domain' has the following mails in one of his mail folders:
- e-mail 1 - from: user1@domain - subject foo
- e-mail 2 - from: user2@domain - subject bar
- e-mail 3 - from: user1@domain - cc: user2@domain - subject baz
- e-mail 4 - from: user1@domain - subject xyz

Now the account 'user1@domain' gets removed from the Exchange server.

After the account 'user1@domain' is removed from the server evolution-ews is used to open the mail folder of 'myaccount@domain'

Based on the EWS_DEBUG logs it looks like evolution-ews currently does the following for this situation:
- Retrieve additional info about e-mail 1
  - Check the local cache to find out if the details for 'user1@domain' are already known
  - Details of 'user1@domain' aren't in the cache yet
    - Perform ResolveNames API call to look up details for 'user1@domain'
    - Server returns 'no results found' as the account doesn't exist on the server any more

- Retrieve additional info about e-mail 2
  - Check the local cache to find out if the details for 'user2@domain' are already known
  - Details of 'user2@domain' aren't in the cache yet
    - Perform ResolveNames API call to look up details for 'user2@domain'
    - Server returns one result
    - Result is saved in local cache

- Retrieve additional info about e-mail 3
  - Check the local cache to find out if the details for 'user1@domain' are already known
  - Details for 'user1@domain' aren't in the cache yet
    - Perform ResolveNames API call to look up details for 'user1@domain' (== again ==)
    - Server returns 'no results found' as the account doesn't exist on the server any more
  - Check the local cache to find out if the details for 'user2@domain' are already known
  - Details for 'user2@domain' are found in the local cache, no need to ask the Exchange server

- Retrieve additional info about e-mail 4
  - Check the local cache to find out if the details for 'user1@domain' are already known
  - Details for 'user1@domain' aren't in the cache yet
    - Perform ResolveNames API call to look up details for 'user1@domain' (== again ==)
    - Server returns 'no results found' as the account doesn't exist on the server any more

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In the example mentioned above evolution-ews sends 3 ResolveNames requests to the Exchange server which all ask for more details about the 'user1@domain' account. For these 3 requests the server returns 'no results found' as the account doesn't exist on the server any more.

To avoid sending too many duplicate/unnecessary requests to the Exchange server evolution-ews should perform the ResolveNames request for each discovered e-mail address only once and when this call returns no results evolution-ews should remember this fact so it doesn't have to ask the server again.