Delay when sending mail to multiple recipients

Hello every one,
We've recently switched e-mail providers at our company, and I'm facing a problem when sending an e-mail to multiple (about 7) recipients. When I send such e-mail, there usually is up to 10 minutes delay before it gets delivered to inboxes. Is it normal behavior when sending to multiple recipients, or is my e-mail provider crap?

I must mention that sometimes there is a mailer-deamon when sending a message to myself (the address is not misspelled) - also don't know why

Thank you all in advance.

Can you paste the full email headers from one of the emails? That will show where the delay is.

No, it's not normal for an email to take over 10 minutes because of it having multiple recipients.

It sounds like something is wrong with the email service. Pure speculation, maybe there is a business archiving feature storing copies of all emails that's not working correctly.