Hello Babak M - welcome to Android Forums... thanks for joining!!
Usually, if a message gets too long to be sent as a normal SMS message, it would auto convert to an MMS message. The reason you may not be able to send MMS messages could be that you do not have the correct APN's (Access Point Names) set up for your carrier... you should be be able to find them on your carrier's website (or get them by speaking to your carrier).
I think your phone is the Galaxy Ace - you might like to check in the Samsung Galaxy Ace - Android Forums sub-forum, as other users of the same phone are usually the best to try and help you out with issues with your phone
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