A quotation that lands in a spam folder costs you the job just as surely as one you never sent. Most delivery problems come down to authentication records that were never set up.
The three records that decide your fate
SPF lists which servers are allowed to send mail using your domain. Without it, anyone can forge your address, and receiving servers know that, so they treat your mail with suspicion.
DKIM signs each outgoing message cryptographically so the recipient can verify it genuinely came from you and was not altered on the way.
DMARC tells receiving servers what to do when a message fails the first two checks, and, importantly, sends you reports so you can see who is sending mail as your domain.
All three are DNS records. Most hosting control panels can generate them. Setting up all three typically takes under an hour and is the highest-return hour available in email administration.
Send from your own domain
If your website sends notifications with a from-address at a free mail provider, those messages will fail authentication, because your website is not that provider's server. Send from an address on your own domain, through a server authorised in your SPF record.
The port and encryption trap
One configuration mistake causes a specific and confusing symptom: sending hangs for minutes, then fails, and the mail server shows no record of any attempt.
That pattern means the encryption mode does not match the port. Port 465 expects encryption from the very first byte. Port 587 expects a plain connection that is upgraded partway through. Choose the wrong pairing and neither side receives anything it can interpret as an error, so the connection simply stalls rather than failing cleanly.
The rule is short: port 465 goes with SSL/TLS, port 587 goes with STARTTLS. If your system offers a vaguer choice such as 'TLS', check which one it actually means.
Watch what your own domain sends
Once DMARC reporting is on, you will see every source sending mail as your domain, including systems you had forgotten about and anyone forging you. That visibility is worth having before a delivery problem becomes urgent.