Topic announce no longer sends mails

[CourtArtanis]

Hi,

Recently, the VPS my webserver runs on was migrated to a new IP address. I have changed the configs in WHM and updated the IP addresses with the new one.
Before this migration, my forum sent announcement emails (via "Announce topic" when creating a post) with no problems at all.
After the migration, the emails no longer get sent, and I've run out of things to try. I use SMTP details for the forum emails, but phpmail doesn't work either.

The strange thing is, automated emails such as new member notifications, and activation emails, seem to send fine without issues. It's just the topic announce that does not.
I've tested emails with a mail client, and the address concerned can both send and receive email I send using an alternative personal email.
Is there anything I could have possibly overlooked with SMF?

No settings have been changed other than my dedicated IP.

EDIT: Issue has gone tentative for now.

[Jade Elizabeth]

Do you have any idea why it happened or was it just a weird hiccup? Is it possible the mail server was down or your queue was quite long and delayed you getting the email right away?

[CourtArtanis]

Do you have any idea why it happened or was it just a weird hiccup? Is it possible the mail server was down or your queue was quite long and delayed you getting the email right away?

Thanks for responding
The MX mail queue kept filling up, and I had no idea why. I have switched back to sendmail for now, but the problem with that for me is it seems to have a lower compatibility, as it triggers transparent spam filters on certain ISPs. It has been a while since I've used it as a permanent type though

I was tired last night and kinda ragequitted my desk I need to look at it some more today, and if I find the issue with SMTP I'll post it here, but so far I have no reasonable explanation lol

[Jade Elizabeth]

That's weird, maybe ask your host? I'm not sure it's specifically an issue with SMF if it's filling mail queues!

I didn't know that about the sendmail function, thanks for sharing that!