Why use an in house mail server?

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I have been doing some design work at a local charity, theres probably about 20 people there, and they dont all have email address yet they have a mail server installed on thier network, that handles all thier outgoing/incoming mail. They have had some problems with this system, does anyone know, why they might be using a mail server in house rather than just a providers pop/smtp ? There’s no one to ask there 'cos the person that set it up doent work thier now and is out of the country, just thought I’d ask!! thanks ppl. (-: