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Email is transmitted across networks via Mail Transfer Agents (MTAs). MTAs communicate using the Simple Mail Transfer Protocol (SMTP) and act as both client and server, depending on the situation.


For example, an MTA can act as a server when accepting an email message from an end user's MUA, then act as a client in connecting to and transferring the message to the recipient domain's MTA for final delivery.

MTAs can be described with more specialized language that denotes specific functions:

  • Mail Submission Agents (MSA): An MTA that accepts mail from MUAs and begins the transmission process by sending it to a MTA for further processing. Often the MSA and first-hop MTA is the same process, just fulfilling both roles.
  • Mail Delivery Agent (MDA): An MTA that receives mail from an organization's inbound MTA and ultimately places the message in a specific mailbox. Like the MSA, the MDA could be a combined in-bound MTA and MDA component.

MTAs may also perform various security functions to prevent malicious email from being delivered or include authentication credentials such as digital signatures. These security functions may be provided by other components that act as lightweight MTAs or these functions may be added to MTAs via filters or patches.