Just to be pedantic as it might matter in some context, it's no longer given as an option to email the NOT to the RTB. The ways given in their guidance is upload via online portal or post.
Hmm, I wonder if that is actually allowed

. I think probably, it is.
The Act said:
[The notice] may be served on or given to the person in one of the following ways:
(a) by delivering it to the person;
(b) by leaving it at the address at which the person ordinarily resides or, in a case in which an address for service has been furnished, at that address;
(c) by sending it by post in a prepaid letter to the address at which the person ordinarily resides or, in a case in which an address for service has been furnished, to that address;
(ca) by sending it by electronic means to an email address, fax number or other electronic contact point used by the person for receiving emails, faxes or other electronic messages, where a record that the notice or other document has been sent is made for the sender by the email system, fax machine or other electronic system used;
(d) where the notice or other document relates to a dwelling and it appears that no person is in actual occupation of the dwelling, by affixing it in a conspicuous position on the outside of the dwelling or the property containing the dwelling.
Sub-section (ca) says that you can send it by "electronic means".
Does uploading to a website count.
It would count as "electronic means".
It is not "an email address", or a "fax number".
Is it an "other electronic contact point"? That isn't 100% clear. Is a website a "contact point"? I could see a tenant arguing that it isn't a contact point. Someone might give you their website domain name, but then you would actually contact them using the email address on the website. The email address would be the contact point, rather than the domain name.
OTOH, some people might put their website domain name as part of their contact details on a business card.
In any case, it is not one of the explicitly valid methods for contacting people.
It has to make a record that the notice was sent. I assume that the RTB do that. Can you view the notices you have uploaded on their website.
An interesting point is that if "an address for service has been furnished", you must use that address. Safest would be to send to both the dwelling and the furnished address.
This might mean if there is an agreement to use the email address, then you must use the email address. Though that is a different sub-section, so probably not.