We use a 1and1 package here and it works fine. I've far more trust in 1und1 to manage their servers, backup and fix any problems in the event of a fire, flood, virus, than anyone here!
Yes you are right the normal routing for mail going from Erin@yourco.com
would be to go out to the 'external' mail server, and then for Joe to pick it up which could add bandwidth. If its big files you might want to encourage just moving the files (e.g. network storage). For small files we never noticed a big issue when I had 5 people sharing a 256k upload speed. Our new speeds aren't anything like Willk's but even with dozens of people sharing a 1000 Mb/s upload i've never noticed a problem (we have no bandwidth limits).
By hosting at the office you get two new problems with bandwidth:
(1) Spam. You are going to receive every message for your server. If you use google you'll cut your mail traffic significantly (I would say by > 50%!).
(2) If people connect from home (or via mobile etc) then their access to your mail server is across the broadband connection which will probably be the bottle neck. If you send and email from home to someone you have to upload it to your work server then it has to send it back out again. Don't know how much off site stuff people do.
I think there are ways of having both internal email addresses and external - but its probably more hassle than its worth.