Although this subject is covered in multiple sources, it's hard to find it for some reason or at least it wasn't that easy for me. That's why I repeat it once again here and provide links to more verbose explanations.
What is it for
Some people/e-mail clients do not provide plain text
versions of their messages, which is inconvenient if at least one of recipients
uses something like mutt to read e-mails. Fortunately, mutt is
smart enough to be able to convert such e-mails to plain text format on the
auto_view option and
mailcap_path option with
mailcap file). The issue is that once automatically viewing of
HTML is set up mutt starts to open HTML version of e-mails even when there is
plain text version available. HTML converted to text lacks hyper-links to
follow using something like urlview and, in general, doesn't make
much sense, plain text suits for emails better than HTML.
Really short answer
alternative_order setting. Excerpt from
alternative_order type[/subtype] [ ... ] unalternative_order [ * | type/subtype] [...] alternative_order command permits you to define an order of prefer‐ ence which is used by mutt to determine which part of a multi‐ part/alternative body to display. A subtype of “*” matches any subtype, as does an empty subtype. unalternative_order removes entries from the ordered list or deletes the entire list when “*” is used as an argument.
auto_view text/html alternative_order text/plain text/html