On a BigV server I run email for a charity event (https://www.echtrees.org.uk). Outgoing email is very low - say 10 - 20 per day. 4 or 5 times a year we send out mailshots to approx 8,000 supporters. It looks as if a large proportion of these are marked as spam by the large email providers. It is likely that this is causing us to lose funds for the charity.
We suspect that the cause of this is that the mail providers see the sudden volume increase as suspicious. We do try to meet the recommendations such as SPF, DKIM, DMARC.
Can anyone suggest a way out of this?