Transactional emails.
Every time I meet for the first time with a SaaS Company, I ask a list of questions to see where they are at.
One question is "How are you sending out your Transactional Emails to your customers (and trials, freemium users)?
The answer I often get is that their engineering team is sending out emails directly from the product (for password resets, product messages and the like).
While they sometimes use MailChimp or another simple marketing automation tool for their newsletter, they have not made the step to use the transactional email sender equivalent (mandrill, mailgun, SMTP sender, send grid or Amazon SES are some examples).
This is a mistake. Make sure to use reputable providers to send out both marketing emails and transactional emails from day one. When your volume is still low this is not a big cost and non-delivery challenges will cost you much more down the road.