Environment
Content Manager 9.x 
Content Manager 10.x
Office 2016 

Situation
When using Send to email function, the default font will always be Times New Roman regardless of settings. This happens when the compose email setting in Outlook is being set as 'HTML' (Outlook Options - Mail)

Resolution
To work around it, they should set it as 'Rich text' format. Then the default font will now also be set to Calibri when using Send to mail.

Another workaround is to use the 'Send record to mail recipient' template, however, this way will overwrite other settings, e.g: if we disable the 'Attach a Content Manager reference to the generated email message' in the Email Template, the reference file would not attach or vice versa. Hence to be able to attach TR5 on the user's demand we should not use this template.

Access support article here