Denis, I discussed this with several others and we came to the conclusion that the best fix would be to:

• first show a warning
  "Some tokens have a 'valid from' date set which is in the future. When sending emails now they will not be able to access the survey before that 'valid from' date.
  Do you want to continue?" (Yes/Cancel)
• if YES -> send emails
• if NO -> stop

See https://www.limesurvey.org/en/forum/design-issues/65596-invalid-token?start=10#125967 for forum discussion.

Just a "check box" in send mail , unchecked by default.

Well, I'd really like to see a warning if there are tokens that are not valid yet.
Maybe we can hide the check box and show it after having shown the warning?

All are to be reviewed for 2.05 AND 3.0 . Then just a checkbox.

If only a checkbox we definitely need to adjust the error message! We should change
"Email to XX (xx@xxx.org) delayed: Token is not yet valid."
to:
"Note: Token for XX (xx@xxx.org) is not yet valid."

The "All emails were sent" part I think we can leave as is.

Fix committed to master branch: http://bugs.limesurvey.org/plugin.php?page=Source/view&id=15934

I personally like the first option better:

• first show a warning
  "Some tokens have a 'valid from' date set which is in the future. When sending emails now they will not be able to access the survey before that 'valid from' date.
  Do you want to continue?" (Yes/Cancel)
• if YES -> send emails
• if NO -> stop

It would definitely give more flexibility
Thanks for looking into that!

With your option :

• Send valid email but not send invalid

Aren't possible.

Version 2.06+ Build 150930 released