We have been moving from an Outlook/Sendmail solution to hosted Zimbra solution in our small business but it appears the lack of one facility has killed this off. That is the ability to send mail as someone else from a different domain. Let me explain. We work for CompanyA and directly contrcat with clients as we are happy to send mail as user@CompanyA. Howvere, we also do sub-contrcat work for CompanyB (and C & D etc). Now CompanyB has us registered on their mail system but all mail for us is forwarded to us so neil@companyb is forwarded to neil@companyA. Now CompanyB is really keen this its customers do not know what they sub-contract nor likes us telling customers our own details in case they come direct to us. So we have an agreement that we send all mail out as user@companyB. In our marketplace this is common.
In Outlook (and every other mail client I have looked at) this is a piece of cake you simply set up different accounts with the same server details but with different email addresses and then the account you use to send the mail shows the user the email address you have set for that account. Zimbra does not allow you to do that. You can set-up different personalities which can have different reply-to addresses but it always shows the email address of the the primary account as the sender.
This ability to completey appear if you are somewhere/someone else is vital to our sub-contracts. The only solution seems to continue to use Outlook as a client and then we loose all the collaboration facilities which is a real pity.