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General Notes
* MX is NOT required, but an A record, or CNAME to a MX MUST be present at the least.
* EHLO should be tried, then fall back to HELO
* The 250 return code from RCPT TO is not actually clear-cut. A 251 may be
returned if the message was forwarded to another address. This could be a
useful indicator to end-users that an address should be updated.
* RCPT TO can accpet just "postmaster" without a domain name
* Server MUST not close connection before:
- QUIT and returning 221 response
- Forced requirement, and only after returning a 421 response
- Clients expriencing a forced connection closure, without prior warning should
just treat it like a 451 closure regardless
* ALWAYS use blocking sockets for the initial connection (this should prevent
Exim4's whining about sync).
Response codes
* From RFC2821, 4.2.
- In particular, the 220, 221, 251, 421, and 551 reply codes
are associated with message text that must be parsed and interpreted
by machines.
Error Codes
* Numeric 5yz = Error
- 550/RCPT TO = Relay denied
- 500 = Unknown command
* Numeric 2yz = Normal
* Numeric 4yz = Temporary failure??
class EsmtpTransport implements Transport, EsmtpCommandWriter {
interface EsmtpCommandWriter {
public function getBuffer();
public function executeCommand($command, $expectedCodes);
interface Extension {
public function getKeyword();
public function afterEhlo($commandWriter);
public function getRcptParams();
public function getMailParams();
public function atCommand($commandWriter, $command, $expectedResponse) throws CommandSentException;
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