I can't tell for sure. Odds on is it is an autopen. What gave me some hope that it is original is
1. The signature appears so sloppy when I recall autopens of other presidents. If one is to go to the trouble for an autopen sig, why not take the time to make it readable. Unless, of course, one wanted to make the sig look casual.
2. I compared autographs with those I could find online and I found none with the trailing tail at the end.
Under 10x glass the ink appears thick and solid, but without telltale splotches at the beginning or end as mentioned in the link that Don posted.
What makes you think that when I tell you that this is a genuine signature, that this not the word of an expert in presidential autographs. By the way, who the letter is addressed to means nothing. Singer Pearl Baily's presidential medal of freedom certificate bore a non authentic signature of Ronald Reagan. However, I hate to disappoint you. The value of this letter is around $250-300. COA's in autographs do not attach the same meaning in the autograph field as the PF etc does with stamps. And certainly not PSA.
This is indeed a very unusual reaction for a situation where a poster is told his item is good. Usually such a dismissive response is given in anger when I have to tell a poster that the item is unfortunately not authentically signed.
Who I am is not really important since as you say you don't know any experts in the field and there are really not expertizing organizations for autographs with the same cache as the PF in stamps. Nonetheless there are people very knowledgable in presidential autographs. I just happen to be one of them. If you don't want to accept my opinion, so be it. It is correct nonetheless. It is logical to think that the recipient of a letter is more important than the signature pattern itself in determining authenticity—it is not. On this letter, the signature is good on its own without the need to bring the recipients identity into the evaluation.
It is a nice clean letter with a dark signature. But it is a routine content thank you note and Eisenhower as president is very common in this format which is why the value is not that high. Your conclusion that I don't know what I am talking about just because you don't Know me or that my opinion is just that— an opinion but not necessarily the correct opinion —is unfortunate and now I regret having wasted my time trying to help you out and why I usually hesitate to enter these discussions. If you stick around this forum long enough you will find that there are other members who seem to know a great deal about their area of specialization but don't feel the need to advertise their credentials and yet you can tell by what they say and how they say it that they know what they are talking about. I hope you won't be dismissive of them.