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Note that each of the five smtp class delivery agents has it own way of specifying the A= delivery agent equate. That is so that you can run each on a different port if you so desire (See this section). The smtp delivery agentThe smtp delivery agent speaks SMTP and has the F=mDFMuX delivery agent flags set by default (see Table 20-19 for the meaning of these delivery agent flags). It is a useful delivery agent if you connect to sites that disconnect when they are greeted with EHLO. Although such behavior violates standards, some hosts still run such broken software. In general, esmtp is preferred over this smtp delivery agent. The esmtp delivery agentThe esmtp delivery agent speaks ESMTP and has the F=mDFMuXa delivery agent flags set by default (see Table 20-19 for the meaning of these delivery agent flags). Note that these are the same delivery agent flags smtp uses, but with the F=a delivery agent flag added to enable ESMTP. This is the preferred delivery agent for delivery over networks. The smtp8 delivery agentThe smtp8 delivery agent speaks SMTP and has the F=mDFMuX8 delivery agent flags set by default (see Table 20-19 for the meaning of these delivery agent flags). Note that these are the same delivery agent flags smtp uses, but with the F=8 delivery agent flag added to force sending 8-bit data over SMTP even if the receiving server doesn't support 8-bit MIME. You might prefer to use this delivery agent when forwarding to a central server that does not understand 8-bit MIME, but that can handle 8-bit data. The dsmtp delivery agentThe dsmtp delivery agent speaks ESMTP and has the F=mDFMuXa% delivery agent flags set by default (see Table 20-19 for the meaning of these delivery agent flags). Note that these are the same delivery agent flags smtp uses, but with the F=a and F=% delivery agent flags added. The F=a delivery agent flag enables support for ESMTP. The F=% delivery agent flag causes all outbound email to be queued instead of sent, and not attempted on normal queue runs. This is extremely useful at a site that is polled only for email. A server with only dial-up accounts might be one example, or a server outside a firewall that is not allowed to push mail inward. With F=% delivery agent flag set, destination hosts need to request delivery with the ETRN command (Section 11.8.2.6). The local administrator can also cause delivery to occur with the -qI, -qR, or -qS command-line switches (Section 11.8.2.3). The relay delivery agentThe relay delivery agent uses TCP to connect to other hosts. It speaks ESMTP and has the F=mDFMuXa8 delivery agent flags set by default (see Table 20-19 for the meaning of these delivery agent flags). Note that these are the same delivery agent flags smtp uses, but with the F=a and F=8 delivery agent flags added. The F=a delivery agent flag enables support for ESMTP. The F=8 delivery agent flag forces sending 8-bit data over SMTP even if the receiving server doesn't support 8-bit MIME. The relay delivery agent also uses an L= (L=) setting of 2040. It also does less header rewriting than the other SMTP-based mailers. This is the delivery agent chosen for forwarding mail to the SMART_HOST (Section 4.3.3.6), LUSER_RELAY (Section 4.5.6), BITNET_RELAY ($B), UUCP_RELAY (Section 4.5.8), DECNET_RELAY (Section 4.5.2), FAX_RELAY (Section 4.5.3), and MAIL_HUB (Section 4.5.7). |
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