Large companies also have multiple buildings. So sending mail to another worker is a common practice. For example, if I need to send some financial documents to the auditor in a building down the street, I will use the Inter Office mail service. Some people might say I.O. to make it short.
The term inter basically means between when there is more than one. For example, in the word international, nation is a country, and if you have inter in front of it, we have international, which means involving two or more nations.