Hello,
Ba careful, you let a screenshot of the connection details for your mailbox, you may want to delete it then change your password.
There is no option in AcyMailing concerning the content, it uses it without modification to create the newsletter.
It seems that the text version of the email is taken instead of the HTML version.
I can see that you're using the "POP3(without imap extension)" option to connect to your mailbox, could you try with another method if this is possible?
The IMAP method is the one AcyMailing handles the best (every mailbox is different, and some of them use weird techniques to encode the emails or segment them in multiple parts).
In the next version of the v5 there will be a modification with the "POP3(without imap extension)" connection option concerning the charset encoding (utf-8 for example) and the special characters in attachments names, but nothing has been changed for the text / html versions.
Have a nice day,
Jeremy