This must be the reopened issue QB-2695, but I have no permissions to do it.

I run a task sending email-notification.
Task is "MSBuild.ExtensionPack.Communication.Email" from "MSBuild Extension Pack".
It has several recipients passed as a list with a ";" (semicolon) delimiter and now they contain spaces in them.

So, step is msbuild task, properties are
EmailSender=Quickbuild <quickbuild@server>
EmailRecipient=Recipient1 <recipient1@domain.com>;Recipient2 <recipient2@domain.com>
AnyOtherProperty=Value

run
step fails
16:39:42,171 DEBUG - Executing command: msbuild.exe /nologo /t:sendmail "/property:EmailSender=Quickbuild <quickbuild@server>" "/property:EmailRecipient=Recipient1 <recipient-1@domain.com>;Recipient2 <recipient-2@domain.com>" /property:AnyOtherProperty=Value ...\test.dproj
...
16:39:42,200 ERROR - MSBUILD : error MSB1006: Property is not valid.
16:39:42,200 INFO - Switch: Recipient2 <recipient-2@domain.com>
16:39:42,200 INFO -
16:39:42,200 INFO - For switch syntax, type "MSBuild /help"

But if I run command
 msbuild.exe /nologo /t:sendmail "/property:EmailSender=Quickbuild <quickbuild@server>" /property:EmailRecipient="Recipient1 <recipient-1@domain.com>;Recipient2 <recipient-2@domain.com>" /property:AnyOtherProperty=Value ...\test.dproj
(note a quote before a property EmailRecipient VALUE, but not the option "/property" itself)
it will succeed