Slowing Down Outbound Mail (smtpd)

[Robert B. Carleton]

I have an issue with email providers throttling receipt of outbound mail 
from my MTA. Most of the time, it only handles my email, with maybe 10 
emails being sent outbound a week. All the usual SPF, DMARC, and DKIM 
scaffolding is setup.

One of the sites I host is a non-profit with monthly meetings. There are 
around 50 members. Once a month, a announcement email is sent out. When 
it's sent out, it takes about a day for the queue to clear, because the 
MTA is immediately throttled for sending multiple emails per provider.

I tried setting "smtp limit max-mails" to various values, but I'm still 
getting throttled. Most of the non-profit members have Google, Yahoo, or 
outlook.com accounts. Does anyone have suggestions for better settings? 
I'm running OpenBSD 7.8.

If the emails get sent out over a 24 hour period, that's no big deal. 
I'm just trying to avoid getting marked as an excessive sender. I'd be 
grateful for any suggestions.

TIA,

                        --Bruce