Do not use for Wordpress if at all.
The smtp service itself has intermittent failures that are reluctantly admitted after battering down the support door several times to find out why sites aren't sending emails. Worse still they have a plugin that was heartily sold to me as the Wordpress plugin when I started with them - this was supported gingerly at first, then, it still being on version 1 was entirely disowned by them - despite the plugin having all their branding and their site details and being published and distributed on the Wordpress platforms ... by them. They simply deny it now, having pushed me towards it and supporting it initially. This is not the best solution for Wordpress - in short it doesn't work ... use smtp2go who have their own simple plugin and a working (AND CHEAPER) service. Other solutions are out there but they involve using a 3rd party paid plugin + the SMTP service which works out at over double the price. ALSO nearly impossible to cancel, they literally make you jump through hoops, argue with you and when you explain your issue, deny it and expect you to continue with them for a paltry 5% discount that is still over double the cost of other solutions... OH AND doesn't work !!! Worse, they tie you in, despite the service having failed - they offer no complimentary service, and charge a 30 day cancellation. AVOID!
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