Some people have been reporting problems with mail to gmail backing up. Steve has some information about the problem.
I’m seeing comments in a few places that some gmail MXes (but not all) are tempfailing some pipelined SMTP transactions, which is causing mail being sent there by some smarthosts to back up.
I don’t have details, but if you’re seeing “451 4.5.0 SMTP protocol violation, see RFC 2821” that may be your problem. If you’re running a recent-ish (2.2 or later) version of postfix then “postconf -e smtp_discard_ehlo_keywords=pipelining” followed by “postfix reload” will turn off pipelining altogether as a temporary workaround.
It appears right now to just affecting Postfix, but Steve later says:
I don’t have enough details to know for sure, but I suspect it’ll be hitting most, maybe all, MTAs that use SMTP pipelining at all aggressively. I’d expect most MTAs used by ESPs to be reasonably aggressive with pipelining, as it reduces latency and ups performance a lot.
So it’s definitely worth checking your queues and tempfail logs.
If you’re seeing queues backed up to gmail or you’re seeing that particular 451 rejections from gmail then your smarthost has the problem. If you’re not seeing any problems, it probably doesn’t.