Mac Outlook Subject is garbled and content font size error

My company is using MailPlus server provided by Synology Inc

I encountered two kinds error when using MailPlus with my Mac version Outlook:

1. The subject became "=?utf-8?Q?113=E5=B9=B4=E8=87=B3115=E5=B9=B4=E6=A1=83=E5=9C=92=E5=9C=8B=E9=9A=9B=E6=A9=9F=E5=A0",  it is said we can correct this problem by changing the coding in Outlook.  However, Mac Outlook doesn't allow us to change mail coding if we use 3rd party mail server.

2. The mail content font size is normal when writing and sending from Mac Outlook via Mailplus server. However, it font size may become ten times bigger at the recipient's side.  I have contacted Synology Inc.  They told me it is because that Mac Outlook doens't comply "dot stuffing" according to RFC5321 4.5.2: "Before sending a line of mail text, the SMTP client checks the first character of the line. If it is a period, one additional period is inserted at the beginning of the line." 

Can anyone tell me how to solve this problem?

Thank you.