The Greek word apostolos has 2 meanings. The primary usage in the NT is a special, divinely appointed, eye-witness, Apostle. And is translated 'apostle'.
The secondary usage of apostolos means a regular messenger (used only 3 times in the NT) and it is almost universally translated as 'messenger', 'envoy' or 'delegate', and not 'apostle'.
The BDAG Lexicon entry for the word apostolos helps explains this:
ἀπόστολος,
...
① of messengers without extraordinary status
delegate, envoy, messenger (opp. ὁ πέμψας)
J 13:16. Of Epaphroditus, messenger of the Philippians
Phil 2:25.—
2 Cor 8:23.
② of messengers with extraordinary status, esp. of God’s
messenger, envoy (cp. Epict. 3, 22, 23 of Cynic wise men: ἄγγελος ἀπὸ τ. Διὸς ἀπέσταλται).
ⓐ of prophets Lk 11:49; Rv 18:20; cp. 2:2; Eph 3:5.
ⓑ of Christ (w. ἀρχιερεύς) Hb 3:1 (cp. ApcEsdr 2:1 p. 25, 29 T.; Just., A I, 12, 9; the extra-Christian firman Sb 7240, 4f οὐκ ἔστιν θεὸς εἰ μὴ ὁ θεὸς μόνος. Μααμετ ἀπόστολος θεοῦ). GWetter, ‘D. Sohn Gottes’ 1916, 26ff.
ⓒ but predominately in the NT (of the apologists, only Just.) of a group of highly honored believers w. a special function as God’s envoys. Also Judaism had a figure known as apostle(שָׁלִיחַ; Schürer III 124f w. sources and lit.; Billerb. III 1926, 2–4; JTruron, Theology 51, ’48, 166–70; 341–43; GDix, ibid. 249–56; 385f; JBühner, art. ἄ. in EDNT I 142–46). In Christian circles, at first ἀ. denoted one who proclaimed the gospel, and was not strictly limited: Paulfreq. calls himself an ἀ.: Ro 1:1; 11:13; 1 Cor 1:1; 9:1f; 15:9; 2 Cor 1:1; Gal 1:1; Eph 1:1; Col 1:1; 1 Ti 1:1; 2:7; 2 Ti 1:1; Tit 1:1.—1 Cl 47:1. Of Barnabas Ac 14:14; 15:2. Of Andronicus and Junia (less prob. Junias, s. Ἰουνία) Ro 16:7. Of James, the Lord’s brother Gal1:19. Of Peter 1 Pt 1:1; 2 Pt 1:1. Then esp. of the 12 apostles οἱ δώδεκα ἀ. (cp. ParJer 9:20; AscIs 3:21; 4:3) Mt 10:2; Mk 3:14; Lk 22:14 (v.l. οἱ δώδεκα); cp. 6:13; 9:10; 17:5; Ac 1:26 (P-HMenoud, RHPR 37 ’57, 71–80); Rv 21:14; PtK 3 p. 15, 18. Peter and the apostles Ac 2:37; 5:29. Paul and apostles Pol 9:1 (cp. AcPlTh Aa I, 235 app. of Thecla). Gener. the apostles Mk6:30; Lk 24:10; 1 Cor 4:9; 9:5; 15:7; 2 Cor 11:13; 1 Th 2:7; Ac 1:2; 2:42f; 4:33, 35, 37; 5:2,12, 18, 34 v.l., 40; 6:6; 8:1, 14, 18; 9:27; 11:1; 14:4; 2 Pt 3:2; Jd 17; IEph 11:2; IMg 7:1; 13:2; ITr 2:2; 3:1; 7:1; IPhld 5:1; ISm 8:1; D ins; 11:3, 6. As a governing board, w. the elders Ac15:2, 4, 6, 22f; 16:4. As possessors of the most important spiritual gift 1 Cor 12:28f.Proclaimers of the gospel 1 Cl 42:1f; B 5:9; Hs 9, 17, 1. Prophesying strife 1 Cl 44:1. Workingmiracles 2 Cor 12:12. W. overseers, teachers and attendants Hv 3, 5, 1; Hs 9, 15, 4; w. teachers Hs 9, 25, 2; w. teachers, preaching to those who had fallen asleep Hs 9, 16, 5; w. var. Christian officials IMg 6:1; w. prophets Eph 2:20; D 11:3; Pol 6:3. Christ and the apostlesas the foundation of the church IMg 13:1; ITr 12; 2; cp. Eph 2:20. οἱ ἀ. and ἡ ἐκκλησία w. thethree patriarchs and the prophets IPhld 9:1. The Holy Scriptures named w. the ap. 2 Cl 14:2(sim. ApcSed 14:10 p. 136, 17 Ja.). Paul ironically refers to his opponents (or the originalapostles; s. s.v. ὑπερλίαν) as οἱ ὑπερλίαν ἀ. the super-apostles 2 Cor 11:5; 12:11. The orig. apostles he calls οἱ πρὸ ἐμοῦ ἀ. Gal 1:17; AcPlCor 2:4.—Harnack, Mission4 I 1923, 332ff(Eng. tr. I 319–31). WSeufert, D. Urspr. u. d. Bed. d. Apostolates 1887; EHaupt, Z. Verständnis d. Apostolates im NT 1896; EMonnier, La notion de l’Apostolat des origines àIrénée 1903; PBatiffol, RB n.s. 3, 1906, 520–32; Wlh., Einleitung2, 1911, 138–47; EBurton, AJT 16, 1912, 561–88, Gal comm. 1921, 363–84; RSchütz, Apostel u. Jünger 1921; EMeyer I265ff; III 255ff. HVogelstein, Development of the Apostolate in Judaism, etc.: HUCA 2, 1925, 99–123; JWagenmann, D. Stellg. d. Ap. Pls neben den Zwölf 1926; WMundle, D. Apostelbildder AG: ZNW 27, 1928, 36–54; KRengstorf, TW I 406–46 (s. critique by HConzelmann, TheTheol. of St. Luke ’60, 216, n. 1), Apost. u. Predigtamt ’34; J-LLeuba, Rech. exégét. rel. àl’apostolat dans le NT, diss. Neuchâtel ’36; PSaintyves, Deux mythes évangéliques, Les 12apôtres et les 72 disciples ’38; GSass, Apostelamt u. Kirche … paulin. Apostelbegr. ’39; EKäsemann, ZNW 40, ’41, 33–71; RLiechtenhan, D. urchr. Mission ’46; ESchweizer, D. Leben d. Herrn in d. Gemeinde u. ihren Diensten ’46; AFridrichsen, The Apostle and HisMessage ’47; HvCampenhausen, D. urchristl. Apostelbegr.: StTh 1, ’47, 96–130; HMosbech, ibid. 2, ’48, 166–200; ELohse, Ursprung u. Prägung des christl. Apostolates: TZ 9, ’53, 259–75; GKlein, Die 12 Apostel, ’60; FHahn, Mission in the NT, tr. FClarke, ’65; WSchmithals, TheOffice of the Apostle, tr. JSteely, ’69; KKertelge, Das Apostelamt des Paulus, BZ 14, ’70, 161–81. S. also ἐκκλησία end, esp. Holl and Kattenbusch; also HBetz, Hermeneia: Gal ’79, 74f (w. additional lit.); FAgnew, On the Origin of the Term ἀπόστολος: CBQ 38, ’76, 49–53 (survey ofdebate); KHaacker, NovT 30, ’88, 9–38 (Acts). Ins evidence (s. e.g. SIG index) relating to theverb ἀποστέλλω is almost gener. ignored in debate about the meaning of the noun.—DELGs.v. στέλλω A. EDNT. M-M. TW. Spicq.