SMTP/POP is it best to use 1 domain?

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Is it best to use one domain for all email or create an email domain for
each web site domain?
For instance, lets say my hosting server is domainMaster and there are 10
web sites being hosted on this server.
For simplicity lets say the web sites are company1.com, company2.com, etc.

So is it best (preferred) to create the pop email accounts on mail.domain
domainMaster.com and then alias to each company domain?
Or do you prefer to create an email domain for each company?

I see hosting companies do it both ways and I want to understand the
advantages/disadvantages for each.

Hope this makes sense.

Thanks,
Steve




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