Because I was intrigued by the issues mentioned in this post, I took a while to test a fresh WordPress 2.2 installation's reliability for sending email. I set up a new site on a (gs) Media Temple server with a brand new 2.2 WP install and made the following modifications:
(1) First I installed the ShiftThis.net | Swift SMTP plugin (http://www.shiftthis.net/wordpress-swift-smtp-plugin/) and set it up to send mail via the Media temple SMTP server that goes with the domain. Don't forget this is used, in principle, only TO SEND mail, so it is in no way inconvenient if I don't use my hosted Gmail. Using the latter, however will work but will result in any outgoing mail being logged in the "Sent" folder in the Hosted Gmail account, which may or may not be desired: choose accordingly. I configured my Gmail hosted Premier account to fetch any mail from this account using SMTP, just in case someone replies to an email sent using the actual address designated for accessing the SMTP server. I tested the plugin with Gmail Hosted also, and it worked too.
(2) Second, I configured the SPF records for the Media Temple domain used as an SMTP server (just to make sure this is important, I tried sending an email from the domain using the SMTP server without having set a SPF record : it was sent safely but ended up in my spam folder and in Gmail or Hotmail would have been vanished without trace) I used the Media Temple default for this (v=spf1 a:yourdomain.com/20 ~all) but this can be refined if that mail server is used in other contexts.
(3) I altered the pluggable.php file (found in the wp-includes folder) to replace "firstname.lastname@example.org" by "email@example.com". This had best match the administration default set in the WordPress admin under >Options >General. This ensures that any admin-related mail sent via the SMTP server to myself arrives safely. It is important that this admin address is NOT the same as the address used to author posts, as WordPress does not seem to like that combination.
(4) The last step, obviously, was installing a contact form (I used the WP-ContactForm: Akismet Edition [http://www.bloggingexpertise.com/plugins/wp-contactform-akismet/], basically because I liked the Askimet antispam feature and because it is fairly recent and works, but actually any contact form will do) and set it up to send any INCOMING messages to my Hosted Gmail Premier account. These arrived safely because Gmail established that the SPF record for the domain from which they had been sent specified that server as a legitimate author for that email.
(6) With some plugins other than the Askimet Contact Form I mentioned (which will work with the ShiftThis plugin out of the box), it can be necessary to make the following change (detailed in the documentation for the ShiftThis.net | Swift SMTP plugin [http://docs.shiftthis.net/wordpress_swift_smtp_plugin]; if you’ve installed a plugin that sends email and does not seem to be working correctly with the Swift SMTP Plugin, do the following:
i. Open the problem Plugin in a text editor.
ii. Do a search for the function mail( and replace it with wp_mail(.
iii. Upload the revised plugin and it should now use the SMTP setting for sending mail. (depending on which subscription plugin you're using, it might be necessary to edit it in this way; I needed to edit mine).
Basically the key point is that if you set up your WordPress to send mail using a mail server that Gmail, with its strict anti-spam policies, regards as suspicious because the server it was sent through cannot be authenticated, the chances are the email will not arrive. This applies to both INCOMING mail (sent to you via the contact form) or OUTGOING mail (sent by the WordPress admin to you or to your readers or yourself regarding subscriptions, comments and registrations and the like) and anyone whose email provider has a strict spam policy will either not receive it at all or find it ends up in his spam folder.