You can give admin1 full access to user1 mailbox and allow him to send emails as user1. user1 can be shared mailbox such as Sales, Support, etc.

You can do this either in Exchange Management Console (EMC) or Exchange Management Shell (EMS)

Option 1: EMC

1. EMC > Recipient Configuration > Mailbox [use1] > Right Click > Manage Send As Permission > Add.
   Find admin1 > OK > Manage
2. EMC > Recipient Configuration > Mailbox [user1] > Right Click > Manage Full Access Permission > Add.
   Find admin1 > OK > Manage

Option 2: EMS

1. Add-MailboxPermission -identity user1 -User admin1 -AccessRights FullAccess

2. Add-ADPermission "User1 Name" -User admin1 -AccessRights extendedright -ExtendedRights "send as"
   "User1 Name" - full object's name (or distinguished name), not alias.

After steps in either option are performed admin1 should immediately have access to user1 mailbox, however Send-As permission may not kick in for an hour or two.

Exchange 2007

Created a new distribution group on Exchange 2007 or SBS 2008. Internal emails are delivered, but external rejected with bounce "550 System error".

Problem may be caused by "Require that all senders are authenticated" option in distribution group properties.
Exchange Management Console (EMC) > Recipient Configuration > Distribution Group > [name] > Properties > Mail Flow Settings > Message Delivery Restrictions - Require that all senders are authenticated
If you want to receive external emails this option must be unchecked.

The same option could also prevent external email delivery to user mailboxes and other exchange objects.

MS Exchange 2007
MS Small Business Server 2008

Windows Server Update Services (WSUS) data on Microsoft Small Business 2008 by default is placed in C:\WSUS\WsusContent
This folder can get very large over time, however on SBS2008 you can easily move downloaded data to another drive.

Open Windows SBS Console > Backup and Server Storage > Server Storage
In Tasks Panel click on Move Windows Update Repository Data
Select hard drive you want to move data to and click Move.

This doesn't move actual database (normally located in C:\WSUS\SUSDB), just the downloaded content. However it should be fine as normally database will occupy less space on the hard drive that actual content.

Windows Small Business Server 2008

Exchange 2007 or SBS 2008 suddenly stops receiving external emails.
Internal and outgoing emails are working fine. Emails are not bouncing back. Exchange Management Console seems to be OK. Exchange stores mounted and all Exchange services running.

This behaviour could be caused by a low space on one of the hard drives. Disk doesn't have to be completely full, just getting close to this.
Check Application logs for following events:

Event ID: 15004
Level: Warning
Source: MSExchangeTransport
Resource pressure increased from Medium to High.
Resource utilization of the following resources exceed the normal level:
.....


Event ID: 15006
Level: Error
Source: MSExchangeTransport
The Microsoft Exchange Transport service is rejecting message submissions because the available disk space has dropped below the configured threshold.
Resource utilization of the following resources exceed the normal level:
...

Simply free up some space on the hard drive in question and email delivery will resume. On Small Business Server 2008 the first suspect would be WSUS database.

Windows Small Business Server 2008
Exchange 2007