Once upon a time, selecting an ESP was easy. Open a GSuite account to use Gmail and get on with it.
However, around February 2019, Google introduced TensorFlow (we'll come back to that later) and for about a month it was a total mess (I guess that's what machine learning is, you have to learn it first).
People's accounts were getting locked, even on the first email they sent. They contacted Google support, which didn't work, and many people started looking for other available options.
That's when I started looking into Outlook. As it bahrain mobile database turned out, Outlook enjoyed much better deliverability than Gmail.
The landscape had changed and many lead generation agencies were running solely on Outlook.
As of today, Outlook still outperforms Gmail (even doubling in some cases), although the gap has narrowed since last year.
Outlook deliverability beats Gmail even when contacting another Gmail inbox! Gmail to Gmail deliverability is worse than Outlook to Gmail. Outlook contacting another Outlook inbox has amazing deliverability (unlike Gmail to Gmail).
So, if you're willing to put up with the horrendous setup, then Outlook is the way to go for cold emails.