I am the owner of a company that provides, among several other services, e-mail & web hosting. We currently host about 300 e-mail domains, and about 250 websites. When I started in 2000, my Linux was a RedHat 7.3 & I ran sendmail for my mail clients -- because it was what I already knew, having been a Solaris / AIX / HP-UX / IRIX admin for many years (in fact, my first Unix admin was on DEC Ultrix back in 1987!).
I switched to QMail in 2003 or 2004 (not quite sure which) because it was faster to have an SPF plugin, and I was tired of having to research and test extensively to make each little config change in sendmail (and I had YEARS of experience, not only administering sendmail, but TEACHING IT with Learning Tree as well!)
At first, I did the old-timer trick of just installing QMail (ok, netqmail) and hammering out everything myself (you can't support what you don't understand!) But the more I kept at it, the more I realized that the e-mail "world" was changing faster than I had time (or inclination) to keep up with... DomainKeys, SenderID, DKIM... they were piling up on me, and I had other things to tend to --- like building a service company from the ground up!
Eventually, I stumbled upon the QMail Toaster project -- the folks there work as a team to implement an array of QMail features (the one still missing is DKIM, although DomainKeys works fine). They have chosen what they consider to be the best (or most common) addons to QMail (like VPopMail, CourierIMAP, clamav, spamassassin, squirrelmail, and many more!).
After working with the QMail Toaster for a few months, I happened upon the QMail Toaster Plus (QTP for short) project, which filled in a few holes (like making reading log files & managing queues easier), and most importantly providing a YUM repository (I switched from RedHat to CentOS a while back) for handling updates.
Now, mind you, there isn't a whole lot of updating going on to QMail -- it's actually all those anti-SPAM and Anti-Virus programs that keep me (us) on our toes... and having those updates come via yum has been a blessing to me.
Now, so far I've been looking at QMail vs. SendMail as an admin -- but your question seems to be a request to know more about the user experience. I can tell you that I rarely have problems reported from my e-mail users that are not user error (like forgetting the .com in the address!). When I do, it is usually (by a far margin) a problem with the remote end -- like their being on a blacklist, or not allowing an attachment of the size they're sending. In fact, the one issue that took me the absolute longest to resolve turned out to be another ISP misconfiguring their own sendmail server -- and after I helped THEM debug their own sendmail config (I was personally shocked when they sent me their sendmail.mc file for review!), they switched to QMail 3 months later.
So, as a user, you should see no difference. Messages get through either way, and even very large QMail installations do not delay (or overburden) their servers -- on the contrary, depending upon whom you ask, QMail is now the 2nd most popular MTA on the Internet! (sendmail is still king, if only because it used to have 100% of the market! And the other MTA often touted as #2 is MS Exchange -- good luck getting that to run on Linux!)
So.... I hope I've given you enough information... IMHO, if you're thinking about switching ISPs or hosting companies, I wouldn't give much weight (on the user end) to which MTA software is chosen. BOTH work, work well, and have a long history of service on the Internet. No worries, either way. I'd look for other determining factors (like speeds, uptime guarantees, backup policies, and the like).
Good Luck!
Dan
IT4SOHO
