Most business email trouble is not the mailbox; it is three DNS records nobody set. SPF says which servers may send as you, DKIM signs each message so it cannot be altered on the way, and DMARC tells the receiving side what to do when either check fails. Set correctly, your invoices stop landing in spam and nobody can send as your finance team.
We set all three on the first day, move DMARC from monitoring to enforcement once the reports are clean, and keep the records under version control with the rest of your DNS.