Don’t use text boxes which are drawing objects if they should contain important textual data related to your document. Your name and address is not some fancy logo or decoration. It is a kind of signature which needs formatting. This can’t be done with text boxes. You should use a frame.
A frame is a kind of subdocument where all formatting capabilities (through styles notably) are available. A frame can be anchored to some paragraph and positioned in a variety of ways.
Your personal address may go in the footer of the first page (and then needs no frame if it uses the sheet full width under a separator line). It can also be repeated in every page footer as a kind of stationery. To have it in a rectangular area somewhere on the first page, e.g. in the left margin, put it into a frame anchored in the first footer and offset it, making sure you provide some wrap spacing so that text never overlaps.
The addressee field can be managed with styles if it appears in a conventional sequence like date - addressee - topic - references then text. There, the absolute position doesn’t matter but the various parts must occur in the correct order.
The problem is different if your envelope has a transparent window and you want the addressee paragraph to show through the window when the sheet is folded. Once again a frame is your frame. Since the addressee paragraph will occur just once, anchor the frame to the header paragraph of the first page (only) and offset the frame to the correct position.
A frame can be constrained to a fixed size so that you have a visual feedback in case of overflow. Use dedicated paragraph styles to format the items you enter in this frame. You should never have to use tabs or empty paragraphs to position your data. Styles parameters are there for this role.
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