For larger size companies, a mail room is provided where each employee has their own box. Here are some sentences you might need to know.

"Where is the mail room?"

"I don't have a mail box. Who should I contact to get one?"
"Your mail box was pretty full. You should go pick them up."
"I haven't checked my mail in 2 weeks."
"I never receive anything so I don't need to check my mail often."

"What time does the delivery person usually come?"
"What time does the mail usually come?"

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.

"I can send it to you through Inter Office mail."
"I'll send it through Inter Office mail immediately."
"I'll IO it to you."
"I need those documents. Can you IO it to me?"
"Would you like to pick them up or should I send it through Inter Office mail?"

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.