E-mails sent as HTML convert into "Plain Text" on receiving machine

Sent e-mails convert into “Plain Text” when received.
My Machine: Gateway DX4300; Win-7 Pro; EMClient 5.0.18025.0 Up to Date.
Receiving Machines: #1) HP Pavilion p7-1423w; Win-8; Outlook 2003.
#2) not sure of OS ; Outlook 2003.
I send e-mails in “HTML” format. When they get to the receiving machine, they change to “Plain Text”.
E-mails from other people come in normally as “HTML” messages. I have visited the #1 machine and have been unable to find any way to view the messages as “HTML”. I’ve looked at every configuration setting in Outlook 2003. These e-mails that changed to “Plain Text” were viewed one time as “HTML” and then later they changed to “Plain Text”.
Since this is occurring with 2 seperate machine, the suspicion falls on my machine. So far, no one else is complaining of the issue. Perhaps it’s a combination of EMClient with Outlook 2003.
Any ideas?

Could you please send me one of the HTML messages sent from your PC? Go to the Sent folder in eM Client and save it as .eml. Thank you.

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