Many Thanks fo your answer. As I yet wrote, I'm not a developer and, unfortunately, my knowledge does not permit me to help more by looking at the codes in detail. But I'm here in a proactive way, ready to help and looking for a solution. Vice-versa i am a polite Joomler from its birth, august 2005, so I'm sorry you were offended.
Looking for what I called a "BUG", from wikipedia: "...an error, flaw or fault in a computer program or system that causes it to produce an incorrect or unexpected result, or to behave in unintended ways...".
Just Gmail, not Outlook, is used by 1.8 billion people, around the 80% of users, 90% with Outlook.
I think these are good reasons to justify an investigation.
"... I really don't like it when you brand something as a bug in AcyMailing to obtain assistance..." = I'm sorry, but I never branded as a bug in AcyMailing, I wrote: "...I described in detail the A to F points of because it seems to be just an AcyMailing issue with the auto-fit feature (point C it seems to be also on Outlook) OR, maybe, all the other Mailing Sender have added a workaround for it (I have no the knowledge to investigate it well, sorry)...". I based my deductions in special on these four tests:
C - @baraka has tested it also with the Outlook App
D - I'm having back the same issue with Gmail on my Smartphone, but not on my Tablet (the same Gmail version)
E - If I rotate horizontally my smartphone ("bigger width layout") the issue disappear and the email is auto-fitted (bigger fonts)
F - I'm having back the issue only with the emails received through AcyMailing
About the manual deactivation of the Gmail app option - "...the point 2 could be just a temporally workaround (waiting instructions from the Team I have not yet tested them), because unselecting the "Auto-Fit messages" disable the Gmail "pinch to zoom" email feature..."
What do you think about the Chou558 suggestion ? (I'm not able to test it because I didn't understand how to apply it)
<meta name="viewport" content="width=device-width, initial-scale=1"/>
Then set the style attribute on an image (1px gif works) that spans the full width of the page (style="min-width:600px;"). Everything will be laid out correctly, and it'll be scrollable.
Gmail in general doesn't support media queries, or the style tag in general, which is why we have to move everything inline.
Please, Can that workaround be able to fix also this text/span/dimension "BUG" ?