How do we keep our invoices from landing in clients' bulk mail folder?
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The best way to do this is to ask your clients to "white list" the e-mail address that the invoices come from. They can do this with the help of the IT department or, for smaller companies, the hosting company will be able to help create rules in their e-mail software to eliminate this problem.
You might also want to look into an online invoicing system in which an actual invoice isn't delivered, but a link to an online invoice is.
Hope this helps,