I agree with the earlier comment; I think this phrase is too informal for an email to customers.
That said, I think this expression would be fine in an email to coworkers and colleagues, where a degree of informality is more acceptable. However, I'd probably use a dash instead of a comma:
If you have any feature suggestion then let me know – I'm all ears.
or I would leave out the "let me know" part, because that seems to be a bit redundant in this context:
If you have any feature suggestion, I'm all ears.
Back to your email to the customers – I think you are trying to convey an eagerness for their input. This could be accomplished by adding one word – the word please – to your initial statement:
If you have any feature suggestion then please let me know.
2I'm all ears is classified as informal on thefreedictionary.com. If you're e-mailing customers, you should stick to a more formal register and try to rephrase what you're saying. – Vlammuh – 2015-06-13T09:25:45.567
2Welcome to ELL Stack Exchange! It probably wouldn't hurt to add some more details on who your customers are, as well as their culture and the culture of your company. The answer could easily be different depending on the country. – DCShannon – 2015-06-13T09:51:12.107
1It depends on your company's brand image and the tone of your communications in general; the CEO of Mail Chimp might very well say they were all ears. – Tᴚoɯɐuo – 2015-06-13T11:36:12.060
It's not really that it's too informal; it's that it's the wrong metaphor for the medium. If you say "I'm all ears", it helps to be in the same room (within earshot) of the people you're saying it to! – Brian Hitchcock – 2015-06-13T12:33:38.637
1@Brian Hitchcock: that's taking the idiom rather literally, no? Would you object to his writing "We'd like to hear what you have to say"? – Tᴚoɯɐuo – 2015-06-13T12:46:23.030
No, I'd just assume they were planning an in-person meeting. – Brian Hitchcock – 2015-06-14T07:41:36.927
Thanks for the help guys!! Since I dont really know my users that well (yet), I guess sticking to a formal tone will be more approprirate – Ankit – 2015-06-15T07:14:38.370