Forrester said users were satisfied with the software's reliability. "The general consensus is that the product is reliable - servers generally don't crash or suddenly stop processing mail," said senior analyst Erica Rugullies.
Rugullies said the product has been proven to offer enterprise-class scalability and performance. But she said IT managers should be aware of issues in areas such as calendaring and administration.
"Calendaring is one of Exchange Server 2003's biggest weaknesses," she said.
Rugullies said users had complained of poor synchronisation with the free/busy scheduler, unreliable calendar delegation, double-booking issues with shared mailboxes, and the fact that frequent diary changes sometimes erase meetings.
Microsoft plans to address these issues in the next major release of Exchange Server, expected in the second half of 2006. In the meantime, Forrester advised users to report any problems to Microsoft so that it can hone its service packs.
Rugullies said that for large companies the most cost effective way to license the software was to get Software Assurance on Enterprise Agreements with Premier Support.