Delayed email (sent through QtmmSendMail, either directly or via MMail)

I think I've brought this up before. We have lately been having issues with automated email from our AS/400 being delayed. It's all being sent via QtmmSendMail: one program calls it directly, from RPG, while two others use Giovanni Perotti's MMail front-end, from CL.

This morning, this happened with a "server status" email that goes out twice a day, to three different target addresses, using three separate calls to MMAIL/EMLPTUMSG. The first and the third (to my work email address, and to my personal email address, respectively) arrived with little or no delay.

The second one (to my boss's work email address) was delayed over half an hour.

We have our work email through GMail.

Near as I can tell, in previous investigations, everything makes it out of the building just fine, and our best guess was that graylisting software on the receiving end was the source of the delay.

But in this case, both my work email (no delay observed) and my boss's work email (over half an hour delay) are through GMail, so why should one be delayed and the other not, especially since these same emails have been going out, to the same three target addresses, twice a day, seven days a week, for almost six years?

Anybody have any ideas of where to look?

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JHHL