[4] Lamb later introduced the singer to production duo Future Cut, with whom she had written and produced demos, which were sent to various labels.

[5] Taking advice from Lady Sovereign, the singer created an account on MySpace and began posting demo songs in November 2005.

[5] By March 2006, they attracted thousands of listeners, and 500 limited edition 7-inch vinyl singles of one of the demos, a song titled "LDN", were rush-released and sold for as much as £40.

Musically, "Smile" is a bubbly, mid-tempo tune with "a barroom piano lick", subdued horns and a reggae beat, singing in a light falsetto,[9] while the organ riff contains a sample of Jackie Mittoo playing keyboards on the 1960s rocksteady song "Free Soul" by the Soul Brothers, also written by Mittoo.

[11] It was described to have a "cod-reggae groove that smoulders like a barbecue",[12] as a guitar and piano were used for the background music, following the notes Gm—F as its basic chord progression.

[9] The inspiration for the song came from a real life experience, when Allen broke up with her then boyfriend, Lester Lloyd, resulting in a drug overdose and hospitalization for her depression.

[15] Allen said she later regretted the direct approach of her lyrics: I'm now less inclined to do that, because everything that I do say gets repeated in a way that I haven't said it, or taken out of context and spun in some negative way — and it makes me really sad.

According to Heather Phares of AllMusic, the song "has a silky verse melody that just barely conceals [the singer's] spite", while she keeps "her revenge sweet, the extra sting being given to it by the way she sounds like she's singing about how ice cream or puppies or being in love makes her smile".

[17] Blender reporter Jon Dolan claims that Allen "deploys a sugary melody as a Trojan horse for a smackdown on a douche-bag ex-boyfriend",[18] as Rob Webb from Drowned in Sound called "Smile" an "infectious slice of bouncing, carnival reggae that punches hard with its opening line: 'When you first left me / I was wanting more / But you were fucking that girl next door / What you do that for?,'" and went on to say that the theme of the song is melancholy, "set against breezy beats", and while not being "an obvious TOTP contender on the surface, [it] is good but far from one of the LP's choice cuts".

[20] Dom Passantino of Stylus Magazine suggested that "'Smile' gets burned off the lights by both Sean Paul and Abs when it comes to facsimiles of 'Uptown Top Ranking,' but neither of them could bring the quality of lyricism the singer does,"[21] while Slant Magazine reporter Sal Cinquemani was baffled as to why the song, which she "sings without a smirk of irony", is a UK chart-topper.

[22] The reviewer from NME considered that the song sashays along with sass, while still remaining charming, and said that though it doesn't mark Allen out as excellent dating material, as a soundtrack to the summer, "it’s a dead fackin’ cert".

[12] Adrien Begrand of PopMatters called "Smile" just as good as "LDN", "its loose reggae arrangement augmented by the clever sample of Jackie Mittoo’s piano from the Soul Brothers’ 60s rocksteady tune "Free Soul", as Allen sings bitterly about her ex, with just a hint of vulnerability at first, before going to her friends for reassurance, and confronting the guy during the chorus with a mean-spirited confidence that has us cheering inside".

[10] While John Murphy of MusicOMH praised the song and its "gently lilting reggae rhythm",[23] Priya Elan from NME considered that the Althea & Donna groove of "Smile" is what made fans "fall for her in the first place".

[23] Other reviews came from The Guardian reporter Sophie Heawood, who didn't consider the song as Allen's greatest effort, but still thought she was far better than being called "the female Mike Skinner".

[31] It was her very first single in her home country's main charts, following a top 40 entry with the limited release of "LDN" the same year.

[32] The song shared similar success in Ireland, where it debuted on the issue of 6 July 2006 and peaked inside the top ten at six, holding on the chart for nine weeks.

[38] Despite its low position, "Smile" managed to slowly sell over 500,000 copies in paid digital download and was certified Gold by the Recording Industry Association of America on 9 February 2009; it remained Allen's highest-charting single in the country until T-Pain's "5 O'Clock", which featured Allen, peaked at number 10 in 2011.

[39] Nonetheless, "Smile" remains Allen's highest-charting song as a solo artist in the United States, one of three entries to chart there (alongside "Fuck You" and "The Fear").

There, he tried to explain to her how he was beaten by the muggers, not knowing that, meanwhile, they were breaking down his apartment door and destroying his furniture and possessions, including scratching his gramophone records.

Scavenging through what's left, he happily finds the record box, thinking they are intact, but he suddenly gets diarrhea as a result of the laxatives, but is unable to use his toilet, as it is clogged with his clothes.

As the video finishes, the scene changes to Allen walking down the street at night, smiling and singing the last chorus, while her ex-boyfriend, actually a disc jockey, is in a nightclub getting ready to play his music, but finds out that all his records have been scratched.

But then you have Paris Hilton and the Pussycat Dolls taking their clothes off and gyrating up against womanizing, asshole men, and that's acceptable.

[44]DJ Ron Slomowicz of About.com criticised the music video, saying it was "mean-spirited", it would "alienate any sort of club fan base she might discover", and that if a male "had his friends beating up his ex-girlfriend, trashing her living space, drugging her and destroying her possessions, he would be branded as violent" and would be shunned, thus questioning the singer's taste level as well as her suitability as a positive role model for young girls.

[45] On the day the single was released, Allen appeared on BBC Radio 1's Live Lounge with DJ Jo Whiley, performing an acoustic version of "Smile", and a cover of The Kooks' song, "Naïve".