base64 encoding does not include spaces and short strings are padded (and there is no '@' in RFC4648), so I wonder how this works too.

Could be one of the sibling encodings, like base85 or uuencoded, perhaps? From afar they could look sufficiently similar to base64 that OP got them mixed up?

There are some encodings with @, but the problem is why it would trigger notification in

  aslkdjfhkj@tanelivasdklfjhasdfasdfasd

or in, the best case:

  faskjdhgfkajshdgfasldfhaksdjf@taneliv

In both cases there are something before @ and it's not \s or ^