The MailTask is using the deprecated interface System.Web.Mail to submit e-mails to an SMTP server. Please use System.Net.Mail instead. Here's the reason for why I request doing this:
If you send HTML mails, you have to break long lines (i.e. longer than 998 characters) apart in order to be compliant with RFC 5321 (see 184.108.40.206.6.).
The problem with the old System.Web.Mail is that it does not do this job for you, but the newer System.Net.Mail does. Please note, that having an HTML file with long lines on disk is perfectly fine. However, as soon as you send out this file via e-mail, you have to make sure that you break lines longer than 998 characters apart.
I can develop a patch if you want.