Adverb position in app notification

In our application, when one deletes a room, a notification message pops up. It says "Your room has been deleted successfully". I have been taught that adverbs are often supposed to precede the verb, as in "Your room has been successfully deleted". However the original version with the adverb at the end does sound somehow appropriate to me, although I can't tell why.

Your room has been deleted successfully.

Your room has been successfully deleted.

Which version is preferable? Are they both grammatical?

Your impression that the repetitiveness is helpful is probably why you think the first version sounds preferable. By ending the sentence with a redundant word, you are effectively reiterating the whole sentence. By using the second version of the sentence, you lose this finality on your emphasis word; it just blends into the rest of the sentence. Imagine the reader:

Your room has been deleted (Yes!) successfully. (Yes!)

Your room has been successfully (...) deleted. (Yes!)

In the second example, the user doesn't yet know how to feel about the word successfully because the action hasn't yet been stated. For all he knows, the room could have been successfully duplicated. This causes the success to register only once as he's reading it, whereas the first example registers success twice by the end of the sentence.

Both are grammatically correct, but the first offers more emphasis than the second.

Both orders are okay. The adverb at the end sounds informal, to me. That rule that you've heard to put the adverb before the verb is not a very good rule.

Different types of adverb occur in different positions, and often there is a choice of placement. In The Syntactic Phenomena of English, McCawley gives a comprehensive account of adverb placement based on which constituent the adverb modifies. I think he would classify "successfully" as a V-bar modifier. There are several V-bar constituents in your example, but [deleted] is one of them, and modifiers often go immediately before or immediately after the constituent that they modify.

Zeno Vendler has also discussed adverb classes, and "successfully" might possibly count as a moral adverb in his system. The moral adverbs are those concerning praise and blame, and doing something successfully is praiseworthy. I'm not sure about that classification, though. The moral adverbs are V-bar modifiers in McCawley's classification, so that is consistent with the permissible places of "successfully" in your example, before or after the V-bar "deleted".

I couldn't find Vendler's article with a web search, but my recollection is that it appeared in a Chicago Linguistic Society volume Papers from the Parasession on Natural Phonology (oddly enough).