Mailmerges do not work well with mixed data formats. To a large extent, that is because they are designed to work with databases where only one data format is permitted per field - in cases such as this the postal codes would have to be stored in a text field. With your Excel data source, the simplest solution is to ensure the first few records include data in the alpha-numeric format, so the mailmerge process will interpret it as a text field. You should then be able to do the merges with just:
{MERGEFIELD ZIP}.

See the Mailmerge Data Format Problems topic in the Mailmerge Tips and Tricks 'sticky' thread at the top of this forum: https://www.msofficeforums.com/mail-...ps-tricks.html

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]