Emailing business is mainly about
1. data (good or at least fresh)
2. ips (and maintaining reputation – see #1) [several for Big4 Broadband / 1 a day for GI]
3. MTA supporting current standards (DKIM, SPF, DMARC, ip throttle, message throttle, back offs, etc…)
4. domains (for infrastructure – froms, clicks, rdns, fbl, etc…) some of this doesn’t apply to GI but to Big4 Broadband
As for offers, subjects, format, servers, networks – DO 1-4 first !
The rest is just tweaking and optimizing.
These are also important but without 1-4, it will be an uphill battle.
Even if you have the dumbest subjects in ugly broken english text format
promoting a used condom with a flaky network or advertiser, YOU WILL get opens and clicks
and maybe 1 convert if 1-4 is in place.