You can usually contest in more than one way. They are not equally safe. Here is how the channels compare on the only thing that matters in a dispute: proof that you sent it, on time.
Registered post — the safe default
A registered letter (lettre recommandée / aangetekende brief) is accepted by every Belgian agency and gives you a dated receipt. If there is ever a dispute about whether you contested in time, the postmark settles it. A qualified electronic registered letter (via authorised platforms) has the same legal value as paper. This is the channel that always works.
Online forms — convenient, but variable
Many cities offer an online objection form. They are fast, but they change often, sometimes require an itsme/eID login, and don't always give you a downloadable receipt. Always screenshot the confirmation page.
Email — accepted for some, not all
Email is explicitly accepted for GAS/SAC objections and by some parking services (e.g. Bruges). Where accepted, it leaves a timestamped trail. Where it isn't published as a channel, don't rely on it.
The bottom line
Use the online form or email for convenience if your city offers one — but if your deadline is tight or the case matters, send it by registered post. Quitte always gives you a print-ready letter and the correct postal address, so the reliable fallback is one click away even when an online portal is down.