If CRA has sent you a notice of reassessment, then, whether you got it, it is valid. If CRA sends it to the wrong address, though, it may not be valid unless the address was wrong because you didn't tell CRA about your address change. (See Austin v. The Queen, 2010 TCC 452 (also Woods J.) at paras. 11-13.) It wouldn't be hard for you to show that CRA used the wrong address; though CRA might still be able to show that you got the reassessment notice or that you knew about it.
If CRA has sent the notice to you, then you are deemed to get it on the day CRA sent it: ITA s. 248(7). But what if CRA didn't send you the notice? How do you prove that CRA didn't send you a reassessment notice? How do you know? You know you didn't get it but how do you prove CRA didn't send it? In fact, you needn't prove it. All you need do is ask for proof. CRA must then prove the sending with an affidavit from a CRA officer: ITA s. 244(5).
"[W]here the taxpayer alleges that he has not received notice of the assessment the burden of proving the existence of the notice and the date of mailing is on the Minister." Sometimes, CRA can't give this proof and so the tax courts allow the objection or appeal to go ahead on the basis that the objection and appeal deadlines never started. (See Kovacevic v. The Queen, 2002 CanLII 46882 (TCC, Bonner) at para. 11; And for a review of some cases, see Lambo v. The Queen, (2011 TCC Little) at paras. 29-33.)
All these cases should have helped Mr. Chomatas, except Justice Woods said that "Mr. Chomatas appears to accept that the notice of assessment was sent and so this issue does not arise in this case." (para. 17) This result seems to impose a harsh technicality on an unrepresented taxpayer. But the lesson remains: If you did not get CRA's reassessment notice and you have missed the normal time periods to object, say you did not get the reassessment in your Notice of Objection and if CRA insists on the time limits, ask for proof that the reassessment was sent. (Even where CRA offers proof, courts sometimes find the proof incomplete.)
See Chomatas v. The Queen, 2013 (TCC 319 Woods)