Supported by the singles "Save the Day", "Out Here on My Own" and "Here We Go Around Again", the first of two discs on the album featured a mix of b-sides and previously unreleased material from Carey's discography.

"Basically, I found stuff in my vault that I had either started to work on a long time ago and never released or that I wanted to finish mixing or do whatever.

"[8] Carey channeled the Jackson 5 on this "buoyant bop from 1990, sounding so innocent that she can barely contain the sheer joy of relishing in her own melismatic powers.

[8] Carey's work on "One Night" was also an "urban groove that probably scared the execs at Columbia at the time" and was "one of Mariah’s first collaborations with Jermaine Dupri, a creative relationship that would bear a lot of fruit in coming years.

[17] "I Pray" was described by The New York Post writer Chuck Arnold, as "one of the most straight-up spiritual things Carey has ever done, with a choir helping her to take it all the way to church.

"[9] "Cool On You" continues the R&B sound and "stems from E=MC² era and finds [Carey] in full club mode [...] It's light and feathery fun that gets stuck in your head after the very first listen.

[26] On October 23, 2020, to further promote "Save the Day", Carey released a lyric video for the song which encouraged viewers to vote as well as paying tribute to "Breonna Taylor, Congressman John Lewis, trans activist and writer Raquel Willis, Sojourner Truth, Frederick Douglass, and essential workers on the frontlines of the pandemic, with portraits drawn by artist Molly Crabapple.

"[27] In the weeks leading up to the album, Carey released "digital EPs, remixes, bonus cuts, rare tracks, a cappella renditions and live performances" from her discography under the coined hashtag "#MC30".

[30] To further promote her 30th anniversary and the release of The Rarities, Carey released EP remixes of her songs: "There's Got to Be a Way", "Someday",[31] "Emotions", "Make It Happen",[32] "Dreamlover", "Never Forget You", "Anytime You Need a Friend",[33] "Fantasy", "One Sweet Day", "Always Be My Baby", "Underneath the Stars",[34] "Honey", "Butterfly", "The Roof", "Breakdown", "My All",[35] "Sweetheart", "I Still Believe" and "Theme from Mahogany (Do You Know Where You're Going To)".

[44][45] In 2021, she resumed "#MC30" releases with EP's for the songs "Through the Rain", "Boy (I Need You)", "I Only Wanted", "The One", "Bringin' On the Heartbreak",[46] "It's Like That", "We Belong Together", "Shake It Off", "Don't Forget About Us", "Say Somethin'", "Your Girl",[47] "Touch My Body", "I Stay in Love", "I'll Be Lovin' U Long Time", "I'm That Chick"[48] and "Joy to the World".

"[21] He gave praise to Carey's 2020 rendition of her classic, "Close My Eyes" from her sixth studio album Butterfly stating that, "Her vocals [...] are lovely and understated, even when she’s hitting her legendary whistle notes.

"[21] He also criticized "Save the Day" stating that "Ms. Lauryn Hill’s vocals have been cleverly interpolated" but called it a "rare lyrical misstep" saying that "what the world needs now is something more probing and specific than another string of peace homilies.

"[61] In describing the album, The Cavalier Daily writer Darryle Aldridge stated that it "lets fans and critics into a glimpse of [Carey's] true self, particularly the R&B siren that masqueraded as a tight-laced girl-next-door during her reign in the ‘90s.

[66] The album also debuted and peaked inside the top-twenty in Australia and Spain, the top-thirty in Japan, Scotland and Switzerland, and the top-forty in Belgium, Croatia and Poland.