If you sent me a message and wondered if I've received it, you might want to consider the following:
I usually check my mail once in the morning, and once at night, and respond straight away (if your query is simple to answer). You should, normally, get a reply by the next day (though perhaps not, during the weekend). If this is too long a delay for you, then please get in touch with me via one of the other methods mentioned on my contact details page.
Posters trying to sell something to me are forewarned to read my thoughts about marketeering before attempting to make contact with me.
Messages that seem like spam, even slightly, might get treated as spam (either automatically filtered out, or ignored by me). Don't send spam, and don't send anything even remotely like spam (e.g. similar styles to spam, similar messages as spam, messages that quote spam messages, erroneous addressing similar to spam, messages that look like they're from a nutcase, etc.), and don't send me warning messages about spam (they're spam, too).
If you think that your mail may have been rejected by an anti-spam system, then re-send it, but differently (e.g without HTML, without extraneous files, without anything like spam, with sensible names in the addresses, sensible subject lines, and with the correct date, etc.).
Be aware that as soon as any anti-spam system flags you as being a spammer, you may be permanently regarded as one. i.e. You may never be able to get another message through, again (no matter how you reword it, trying to get past the anti-spam system). The moral is don't do anything so stupid in the first place.
Messages that addressed to numerous recipients, hidden recipients, or otherwise appear to be a form letter fired off indescriminately, mightn't get any response, other than to be regarded as probably being spam.
Messages that are incoherent or just plain weird mightn't get any response.
Messages sent with the wrong date will be filed using that date, and not show up with the current messages. I do not go looking for ancient or far-future messages, and some anti-spam measures flag such messages as possibly being spam. Set your clock correctly.
Messages that don't seem to elicit a response might not get one.
Messages that require detailed responses might take a while to prepare, though you'll probably get a brief note about that, in the meantime. And if you want to work through something really complex, it might be best to phone me.
Your own anti-spam systems can sometimes be overzealous. Check them if you don't receive a reply, and don't make me go through hoops to be able to get through your systems (add my addresses to your allowed addresses whitelist, beforehand—they're displayed on my contact details page). I don't care much for negotiated mail sentries, and certainly won't enter into negotiations if the double-checking routines use a different mail address to the one I was originally trying to send a message to. If you run such mail blocking schemes, have the courtesy to add people to your whitelist before you as them to e-mail you. Be aware that some automatic anti-spam systems can delay mail delivery for hours, or even days. Some even lose mail, completely. If I find it difficult to reply to someone, I'll probably just delete their message, and won't try responding again.
Some message encryption schemes aren't very compatible with other software, and some people use them incorrectly. Though it's usually still possible to write back to the author, even if the message isn't legible (to tell them so).
Messages can sometimes get lost, you might have to resend yours if you don't get a reply when you ought to.
Messages that don't include your contact details can't be responded to. I receive a number of messages from people who do not include their address, or don't write it correctly. I can't reply without that information, and not going to bother to try and guess your name, address, or phone number, from broken information.
Messages have to be correctly addressed to me to be received. If you get my address wrong, your message may be silently deleted (with no warning to you or me). I may sometimes change addresses (e.g. dropping a spammed-to-death account), so be sure to check my contact details page for my current addresses, or use the form on my site to send me a message.