Thanks guys, I'll try the template change again. Official word from IBM below :
Hello,
we do not recommend any changes to standard template as after that the template is not supported anymore. So in case of any troubles, you will need to reproduce the issue using standard mail template.
Anyway I am providing you some instructions from another PMR which were provided from L2:
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1) First make a copy of mail85.ntf or mail9.ntf template file which is present in Domino\Data folder and keep somewhere outside the Domino\Data folder. This will be as a back up of standard mail template. If you wish to revert to standard one in future, you will have backup in hand by this.
2) Open the mail85.ntf or mail9.ntf in Domino Designer
3) Open the Message form
4) Search for Body field and double click on that.
5) Change the font face, size as per your requirement.
6) Immediately to the right of the Body field edit the text properties:
On the paragraph styles tab (the last one) select [None] instead of "DefaultStyleForMail" See it is highlighted using Green square in below picture.
7) Save the changes to this form.
8) Do the above steps for Reply form as well.
9) After doing this changes, replace the design of any affected user's mail file with this template using File-->Application-->Replace Design-->Select redesigned template (mail85.ntf or mail9.ntf) from server.
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Please inform me if this helped you.
And again I need to inform you that redesigned template is not supported anymore.