What does "Convert ... upon sending" mean?

[Cameron Simpson]

On 21Apr2019 14:35, Paul Gilmartin <PaulGBoulder at aim.com> wrote:
>It appears then, to MacOS Mail.app, in my AOL IMAP Sent folder with 
>MIME headers:
>--4SFOXa2GPu3tIq4H
>Content-Type: text/plain; charset=Iso8859-5
>Content-Disposition: inline; filename="ISO8859-5.txt"
>Content-Transfer-Encoding: 8bit
>
>... and the content correctly shows Cyrillic characters.
>        0  16  32  48  64  80  96 112 128 144 160 176 192 208 224 240
>        0  10  20  30  40  50  60  70  80  90  a0  b0  c0  d0  e0  f0
> 0  0                       &   -   Њ   ц   й   я   ь   {   }   \   0
> 1  1                       љ   /   Ћ   a   j   ~   ы   A   J      1
> 2  2                   ђ   њ   Ѓ   Ќ   b   k   s   з   B   K   S   2
>
>But in my Inbox, with similar MIME headers:
>--4SFOXa2GPu3tIq4H
>Content-Type: text/plain; charset=Iso8859-5
>Content-Disposition: inline; filename="ISO8859-5.txt"
>Content-Transfer-Encoding: 8bit
>...
>and the content is definitely not Cyrillic:
>        0  16  32  48  64  80  96 112 128 144 160 176 192 208 224 240
>        0  10  20  30  40  50  60  70  80  90  a0  b0  c0  d0  e0  f0
> 0  0                       &   -   �   �   �   �   �   {   }   \   0
> 1  1                   �   �   /   �   a   j   ~   �   A   J      1
> 2  2                   �   �   �   �   b   k   s   �   B   K   S   2
> 3  3                   �   �   �   �   c   l   t   �   C   L   
> T   3

How are those listings obtained? Might the thing presenting those 
listings be interpreting the iso8859-5 data as the local character 
encoding and thus misrendering?

Cheers,
Cameron Simpson <cs at cskk.id.au>