Best practice: Is there ever a time when you should send an email to your entire list?

Since, @Ashley I. has already answered the question. Great response btw.

I’d like to add a few cents from my end (just in case it’s not missed out) - 

1. Sending emails to all your contacts at once is never recommended, that’s where segmentation comes into play. Segment you entire list into 4 main cohorts:
   1. Core Audience: Users engaged in the last 90 days (Recommended. Active emailing stream)
   2. Lesser engaged audience: Users engaged in the last 91-180 days (Can be included with above under controlled frequency)
   3. Unengaged audience: Users not engaged at all in the last 180 days & between 181-365 days (Risky audience. However, clean & validate them regularly and keep adding a smaller portion with your active emailing stream once a week or once in 2 weeks - 80% active:20%lesser/unengaged)
   4. Dormant audience: Users not engaged with emails at all in the last 365 days and beyond. (Not recommended for email targeting) - Reach out via different channel.
2. Avoid sending emails to any inactive audience (referring to above – unengaged / dormant). Rather, you can plan a re-engagement program for your unengaged audience and reach out to dormant/inactive users from SMS channel for campaigns or plan a profile enrichment program to get their PII updated and re-subscribe for emails.
3. Ensure targeting the clean, validated & contactable users and avoid sending to any bounce addresses. Implement a 3rd party email validation on webforms.
4. Avoid sending in a batch & blast/sudden high volumes approach, which could hamper the reputation and trigger spam filters to filter out your emails.
5. Closely monitor your reputation, spam complaints & delivery errors on Postmaster and Inbox placement rates on deliverability monitoring tool as you incorporate lesser engaged or inactive audience to coverup your whole audience.