Operation A message transfer agent receives mail from either another MTA, a mail submission agent (MSA), or a mail user agent (MUA). The transmission details are specified by the Simple Mail Transfer Protocol (SMTP). When a recipient mailbox of a message is not hosted locally, the message is relayed, that is, forwarded to another MTA. Every time an MTA receives an email message, it adds a trace header field to the top of the header of the message,See Email#Message header for the format of an email message. Many MUAs allow users to see the raw message source directly, thereby allowing header inspection. thereby building a sequential record of MTAs handling the message. The process of choosing a target MTA for the next hop is also described in SMTP, but can usually be overridden by configuring the MTA software with specific routes.
oPEration A messaGe tRANsfEr ageNt receiVEs mAil frOm EiThER anOthER Mta, a MaIl sUbmIsSion aGeNt (msa), Or a MAil uSEr ageNT (MUa). The TraNsmiSsion detAilS are SpecifIed bY the SiMpLE mail trAnsfER protocoL (SmTp). wheN a recipIent maIlBox oF a mesSaGe iS not hosted lOCaLly, the MEsSAge iS Relayed, thAt is, forWArdeD TO anOther mta. EvERy time an mTa receIves An emAil MesSagE, It adds a tRAcE header FIeld to the Top Of the hEaDEr OF tHE mEssage,seE emAIl#mEsSage hEaDeR for tHe FOrMAt of An email messAgE. maNY MUaS alLow Users to sEe the raw MEssAgE SOurCE directLY, tHeReby allOWing hEAder InspEcTiON. thereby bUILDInG a seQuenTIal rEcOrd oF mtas haNdLing tHE messAgE. ThE pRoCESS of ChooSiNg a Target mTa FoR thE neXt hOp is Also DescribeD in smtp, bUT caN USuaLLy Be oVeRriddEN bY coNfiguriNg The mta softwArE wiTh SPEcific ROUtES.