I can’t find a mailing app in LibreOffice like the one in Microsoft Exel
Excel is not a surrogate for ‘Writer’ but for ‘Calc’.
Would you mind to explain a bit more detailed what you try to achieve. Many contributors here will neither know ‘Word’ nor ‘Excel’ from recent experiences. A ‘like in Excel’ may tell them nothing.
Go > ‘File’ > ‘New’ > ‘Labels’.
Read Chapter 5 (subchapter ‘Printing labels’) of the Writer guide availabel from this site. Version 4.2 will do.
(See also this thread.)