A sample SMTP conversation. Blue is transmit, red is receive; all exists within a command-prompt window. The messages received will vary somewhat, but the initial numbers won’t. The greater-than and less-than signs (< >) are required. The below works fine in Cygwin with ‘inetutils’ installed (which includes ‘telnet’), or in CMD with the Windows ‘telnet’ installed. Linuxes obviously have various telnets! ‘putty’ on Windows will do it also, in telnet mode, port 25, just replace the first line with GUI putty setup.
telnet mail.srcdomain.com 25
220 mail.destdomain.com ESMTP
MAIL FROM: <firstname.lastname@example.org>
250 2.1.0 Sender OK
RCPT TO: <email@example.com>
250 2.1.5 Ok
354 Enter mail, end with “.” on a line by itself
250 2.0.0 Ok: queued as xxxxxxx
Awhile ago it came to light that the following needed to be added to SPF records for email-enabled domains using Constant Contact:
include:ccsend.com include:constantcontact.com include:confirmedcc.com
Things have gotten a lot better. The following now covers all of the above:
A very good SPF checker:
Here is possibly the original SPF checker:
To do a deep study of a bounceback, the best tool this writer has ever seen is here:
in the Message Analyzer tab. Just paste every header you have into there and submit, and then read. Lots and lots of excellent information comes up.
It runs in the background on server, desktop, or laptop (Windows, Linux, or Mac), and it translates native Exchange protocols into that which Thunderbird and appropriate add-ins handle very well. In other words, it acts as a live bridge between Exchange protocols and Thunderbird with Lightning, for contacts and calendar. The instructions are thorough and functional.
A few methods:
The first two have free-of-charge tiers.