The problem is undoubtedly related to the fact your document's surrounding logo is an image with a wrap format set to 'in front of text' and grouped with a textbox in which your actual text appears. Try the attached, which uses a table plus cropped header/footer images to replicate your layout.

Attached Files

Carta final valores alza cuotas 2019 (04-01-2019).docx (252.3 KB, 11 views)

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]

I tried using different formats like you, but whenever I mailmerge it they come out wrong like in the attached file. (this is what it shows on Gmail and outlook using your file)

The only way for it to not change its format is by grouping the text box and the image.

i'm still trying to find ways to fix it, but so far, the previous file works whenever i don't abuse the quantity of senders.

Thank you so much for your time if i find another way i'll post it here too

Attached Images

test 01.PNG (39.4 KB, 16 views)