I post here the conclusion from our mail communication for other visitors:
There is a difference between using Outlook with Exchange server or with other servers.
Microsoft Exchange server requires user (EX record) in the Form field whereas
MailMessage uses email address (SMTP record).
If you want to send the MSG from Outlook using MS Exchange you have to specify Exchange user for the specified SMTP address. Basically you need to do the opposite of ExToSmtp method specified in this post.
Please note that Exchange has “a lot of usernames”. E.g. when tested with my account I can specify mail.From = “Lukas Matyska”, “Lukas M”, “lukasm” or “lukas.matyska”; and everything works fine, however firstname.lastname@example.org fails...
You probably have to set mail.From to “TKBUSR9901” or to “Beheer”.
Later we discovered that including both EX username and SMTP address in the From field works (when sending Exchange takes EX username and ignores SMTP address). So in my case I can do:
// EX user, SMTP address
mail.From = "Lukas Matyska, email@example.com";
instead of (which is not working):
// SMTP address with Display name
mail.From = "Lukas Matyska <firstname.lastname@example.org>";