I host a web/mail server for several fraternal clubs with a fairly small membership. I've been sending emails from club members to a distribution app on the server (a script in /etc/mail/aliases) which expands the target email address to send emails, one-by-one to the members on the list. This works fine, except that I have to space the messages out, time-wise, so the actual recipients' mail server do not see emails from a single sender going to 100 of their subscribers, e.g. gmail.com.

In an effort to speed this up, I subscribed to SendGrid as they have the ability to configure a mail relay. However, it turns out that ALL emails sent via the SendGrid reply have to be to/from one authorized domain. That makes it tough for eg PayPay to send message to this server which are then forwarded to a real email address, not on this server.

That's the problem.

Does anyone know of a bulk email service that will act as a pass-thru relay for any messages sent or received by this server?

Where? At home? AWS? Hosting provider?
Depends on where you're hosting, have you tried your ISP? For providers such as gmail you stand a better chance of delivery if SPF and DKIM signing are in place for each domain.

This is a self-hosted mail server on a business ISP: Breezeline (formerly Wowway). They don't have a bulk emailing service. SPF and DKIM records are in place. The bulk emailing is just to people within the Fraternity, but there can be up to 250 emails at a time and sending that many at once to e.g. gmail causes them to regard the sender as a spammer. I though SendGrid would work, but they require and authorized sender list and in this case, any member is authorize to send to all or a subset of all other members. That's not practical.

I am looking for a bulk email sender who could accommodate any message originating at this server. We restrict who can do so internally and do not other permit relaying.

I can do this with the normal mail server setup, but I have to space the message at fairly long time intervals so as not to trigger the suspicion of destination servers.

Check your IP address isn't blacklisted anywhere. Google and Microsoft (Outlook/Yahoo) also provide facilities to allow an IP address to be "registered" to allow it to build up a reputation that also helps in delivery.

It was something like 5+ years ago since I've had to use a bulk mail delivery company and at the time I was last using them they were in the process of being bought by someone else, so can't really comment on any cheap providers.

I was able to solve this problem by specifying an alternate sendmail config when routing messages through the relay and leaving a non-SMART_HOST config as the normal one used by all other system email processing. The complete how-to on this is posted here: https://www.linuxquestions.org/quest...ed-4175713052/