The answer here is…it depends on what your pain points as an email sender are. If you’re technically proficient, sometimes going with an ESP seems like overkill. On the other hand, a good ESP provides help with email creation, list segmenting, campaign reporting, and with email deliverability.
DIY - Has to find an ISP vendor, setup and configure mailserver software. Make sure domain has the proper DKIM, Domain Keys, SenderID, SPF settings enabled. Warm-up IP addresses if not sent from before. IP address reputation is unknown.
ESP: Configuration has been done. IP address reputation is known. You should be advised as to what domain authentication has been done/is appropriate.
2) Email Creation, Uploading & Campaigns:
DIY: Has to code email, validate email coding for text and HTML, configure email lists and email suppression files, and upload email code and email address lists to MTA software which requires technical and design knowledge. Has to create unsubscribe feature and possibly email delivery preferences features. Harder to do field insertion for email personalization. Harder to segment lists and emails for email testing and personalization.
ESP: Probably has in-app pre-created email templates, email validation services for text and HTML, easy email address import and email suppression features. Most likely has unsubscribe page and email delivery preferences features. Easier to do field insertion for email personalization. Easier to segment lists and emails for email testing and personalization. Personnel doesn’t need as much technical knowledge to use service.
DIY: If MTA software doesn’t have deliverability reporting, probably has to rely on Web server analytics to track opens using tracking .gif and/or Google Analytics to track clicks & Web behavior. Would have to compile unsubscribe and bounce reports manually.
ESP: Should have deliverability reports for opens, clicks, and bounces. Would still use Google Analytics to track clicks & Web behavior. Would get easy to access lists for unsubscribes and bounces.
4) SPAM Complaints and Deliverability:
DIY: Spam complaints are dependent on how the ISP deals with them and how bad the complaints are from upstream providers in particular. Self educate on CAN-SPAM. Would have to deal with blacklist maintainers and network operators on a one to one basis.
ESP: Generally proactive in educating users on CAN-SPAM, and most ESPs will kick off service if too many complaints since their overall deliverability will degrade. ESPs usually have devoted deliverability personnel and good relationships with industry organizations, blacklist maintainers, network operators, etc.
Post inspired by this Quora thread.