If you’re having trouble getting internal email working between users on a Microsoft Exchange Server (mine was Exchange 2013, but this should also apply to 2010 and other versions as well), here’s one easy thing to check:
In Exchange Admin Center (Management Console in 2010 and before), edit a user and check their “email address” section. Make sure that in addition to “SMTP:user@domain.com” you also have included “SMTP:user@DOMAIN.local” – in other words, don’t just give them an email address for the outside world, also be sure to include an address for local internal clients & servers to use too! If you don’t, you’re probably going to get bounced-back emails, rejected meeting invitations, etc.
You would think that this would just be created automatically by default when you give a user a mailbox, but it doesn’t always happen, so check that before wasting time looking for more complex solutions!